# The Functor Combinatorpedia

by Justin Le ♦

**functor-combinators**: hackage / github

(*Note:* This post has been heavily revised to reflect the functor-combinators-0.2 refactoring, as of November 2019. For reference, the original post is available on github.)

Recently I’ve been very productive what I have been calling the “Functor Combinator” design pattern. It is heavily influenced by ideas like Data types a la Carte and unified free monoidal functors, but the end goal is slightly different in spirit. The goal is to represent schemas, DSL’s, and computations (things like parsers, things to execute, things to consume or produce data) by assembling “self-evident” basic primitives and subjecting them to many *different* successive transformations and combiners (through combinators, free structures, tensors, and other options). The process of doing so:

- Forces you to make explicit decisions about the structure of your computation type as an ADT.
- Allows you to retain isolation of fundamental parts of your domain as separate types
- Lets you manipulate the structure of your final computation type through
*normal Haskell techniques*like pattern matching. The structure is available throughout the entire process, so you can replace individual components and values within your structure. - Allows you to fully
*reflect*the structure of your final computation through pattern matching and folds, so you can inspect the structure and produce useful summaries.

Like “data types a la carte” and free monad/applicative/alternative designs, these techniques allow you to separate the assembly and inspection of your programs from the “running” of them.^{1} However, the main difference is that here we focus not just on products and sums, but many different varied and multi-purpose combinators — a “zoo” of combinators. The fixed point is *not* the end goal. The actual ADT data types *themselves* are the goal.

This post is a run-down on the wide variety of such “functor combinators” across the Haskell ecosystem — a functor combinatorpedia. To speak about them all with the same language and vocabulary, this post also serves as an overview of the *functor-combinators* library, which doesn’t really define these functor combinators, but rather pulls them all together and provides a unified interface for working with them. Most of these types and typeclasses are exported by *Data.Functor.Combinator*. Of course, the end-goal is to work with these data types *themselves* directly, so not *everything* is meant to be doable with these typeclasses; they only serve to unite some common aspects.

Right now I already have some posts about this general design pattern, “Interpreters a la Carte” in Advent of Code 2017 Duet and Applicative Regular Expressions using the Free Alternative, but I do have some posts planned in the future going through projects using this unified interface. In a way, this post also serves as the “introduction to free structures” that I always wanted to write :)

Please refer to the table of contents if you are using this as a reference!

## Preface: What is a functor combinator?

A functor combinator takes “functors” (or any other indexed type, `k -> Type`

) and returns a new functor, enhances or mixes them together in some way. That is, they take things of kind `k -> Type`

and themselves return a `j -> Type`

. This lets us build complex functors/indexed types out of simpler “primitive” ones. This includes many some monad transformers, free structures, and tensors.

For example, `ReaderT r`

is a famous one that takes a functor `f`

and enhances it with “access to an `r`

environment” functionality. Another famous one is `Free`

, which takes a functor `f`

and enhances it with “sequential binding” capabilities: it turns `f`

into a `Monad`

.

The main thing that distinguishes these functor combinators from things like monad transformers is that they are “natural on `f`

”: they work on *all* `f`

s, not just monads, and assume no structure (not even `Functor`

).

Sometimes, we have binary functor combinators, like `:+:`

, which takes two functors `f`

and `g`

and returns a functor that is “either” `f`

or `g`

. Binary functor combinators “mix together” the functionality of different functors in different ways.

### Examples

If your final DSL/program/schema is some functor, then functor combinators allow you to construct your final functor by combining simpler “primitive” functors, and take advantage of common functionality.

For example, if you were making a data type/EDSL to describe a command line argument parser, you might have two primitives: `data Arg a`

, for positional arguments parsing `a`

, and `data Option a`

, for `--flag`

non-positional options parsing `a`

. From there, you can *choose* what structure of command line arguments you want to be able to express.

For instance, a structure that can support multiple arguments and optionally a single `Option`

would be:

And a structure that supports *multiple named commands* on top of that would be:

You can mix or match combinators to decide exactly what sort of structures you allow in your DSL.

Now, instead of writing one “giant” `runParser :: MapF String (Ap Arg :*: Lift Option) a -> IO a`

function, you can instead just write parsers for your simple primitives `Arg a -> IO a`

and `Option a -> IO a`

, and then use functor combinator tools to “promote” them to being runnable on a full `MapF String (Ap Arg :*: Lift Option)`

without any extra work.

### Common Functionality

Most of these functor combinators allow us to “swap out” the underlying functor, retaining all of the “enhanced” structure. We abstract over all of these using `hmap`

for single-argument functor combinators (“enhancers”) and `hbimap`

for two-argument functor combinators (“mixers”).

```
class HFunctor t where
-- | Swap out underlying functor for a single-argument functor combinator
hmap
:: (forall x. f x -> g x)
-> t f a
-> t g a
class HBifunctor t where
-- | Swap out underlying functors for a two-argument functor combinator
hbimap
:: (forall x. f x -> h x)
-> (forall x. g x -> j x)
-> t f g a
-> t h j a
```

However, for this post, the concept of a “natural transformation” between `f`

and `g`

— a function of type `forall x. f x -> g x`

, is given a type synonym:

Then the type signatures of `hmap`

and `hbimap`

become:

```
class HFunctor t where
hmap
:: f ~> g
-> t f ~> t g
class HBifunctor t where
hbimap
:: f ~> h
-> g ~> j
-> t f g ~> t h j
```

What does it mean exactly when we say that `hmap`

and `hbimap`

“preserve the enhanced structure”? Well, for example, the type `newtype ListF f a = ListF [f a]`

is essentially a list of `f a`

s. `hmap`

will swap out and replace each `f a`

, but it must *preserve the relative order* between each of the original `f a`

s. It must also preserve the *length* of the list. It’s a complete “in-place swap”. This is formalizing by requiring `hmap id == id`

and `hbimap id id == id`

.

You can also always “lift” a functor value into its transformed type. We abstract over this by using `inject`

(for single-argument functors) and `inL`

and `inR`

(for two-argument functors):

```
-- single argument functor combinators
inject :: f ~> t f
-- two-argument functor combinators
inL :: MonoidIn t i f
=> f ~> t f g
inR :: MonoidIn t i g
=> g ~> t f g
```

Finally, in order to *use* any functor combinators, you have to *interpret* them into some target context. The choice of combinator imposes some constraints on the target context. We abstract over this using `interpret`

and `binterpret`

:

```
class Interpret t f where
-- | Interpret unary functor combinator
interpret
:: g ~> f -- ^ interpreting function
-> t g ~> f
class SemigroupIn t i f where
-- | Interpret binary functor combinator
binterpret
:: g ~> f -- ^ interpreting function on g
-> h ~> f -- ^ interpreting function on h
-> t g h ~> f
```

Having the typeclass `Interpret`

(and `SemigroupIn`

) take both `t`

and `f`

means that there are certain limits on what sort of `f`

you can interpret into.

One nice consequence of this approach is that for many such schemas/functors you build, there might be many *useful* target functors. For example, if you build a command line argument parser schema, you might want to run it in `Const String`

to build up a “help message”, or you might want to run it in `Parser`

to parse the actual arguments or run pure tests, or you might want to run it in `IO`

to do interactive parsing.

For some concrete examples of these functor combinators and their constraints:

```
instance Monad f => Interpret Free f
interpret @Free
:: Monad g
=> (g ~> f)
-> Free g a
-> f a
instance SemigroupIn (:+:) V1 f
binterpret @(:+:)
:: (g ~> f)
-> (h ~> f)
-> (g :+: h) a
-> f a
```

We see that `interpret`

lets you “run” a `Free`

in any monad `f`

, and `binterpret`

lets you “run” a function over both *branches* of an `g :+: h`

to produce an `f`

.

From these, we can also build a lot of useful utility functions (like `retract`

, `biretract`

, `getI`

, `biget`

, etc.) for convenience in actually working on them. These are provided in *functor-combinators*.

Without further ado, let’s dive into the zoo of functor combinators!

## Two-Argument

Binary functor combinators “mix together” two functors/indexed types in different ways.

We can finally *interpret* (or “run”) these into some target context (like `Parser`

, or `IO`

), provided the target satisfies some constraints.

For the most part, binary functor combinators `t`

are instances of both `Associative t`

and `Tensor t i`

. Every `t`

is associated with `i`

, which is the “identity” functor that leaves `f`

unchanged: `t f i`

is the same as `f`

, and `t i f`

is the same as `f`

as well.

For example, we have `Comp`

, which is functor composition:

We have an instance `Associative Comp`

and `Tensor Comp Identity`

, because `Comp f Identity`

(composing any functor with `Identity`

, `f (Identity a)`

) is just the same as `f a`

(the original functor); also, `Comp Identity f`

(or `Identity (f a)`

) is the same as `f a`

.

From there, some functors support being “merged” (interpreted, or collapsed) from a binary functor combinator, or being able to be injected “into” a binary functor combinator. Those functors `f`

have instances `SemigroupIn t f`

and `MonoidIn t i f`

. If a functor `f`

is `SemigroupIn t f`

, we can `interpret`

out of it:

```
binterpret
:: SemigroupIn t f
=> (g ~> f)
-> (h ~> f)
-> (t g h ~> f)
biretract
:: SemigroupIn t f
=> t f f ~> f
```

And if a functor `f`

is `MonoidIn t i f`

, we can “inject” into it:

```
pureT
:: MonoidIn t i f
=> i ~> f
inL :: MonoidIn t i g
=> f ~> t f g
inR :: MonoidIn t i f
=> g ~> t f g
```

A more detailed run-down is available in the docs for *Data.Functor.Combinator*.

One interesting property of these is that for tensors, if we have a binary functor combinator `*`

, we can represent a type `f | f * f | f * f * f | f * f * f * f | ...`

(“repeatedly apply to something multiple times”), which essentially forms a linked list along that functor combinator. This is like a linked list with `t`

as the “cons” operation, so we call this `ListBy t`

. We can also make a “non-empty variant”, `NonEmptyBy t`

, which contains “at least one `f`

”.

For example, the type that is either `a`

, `f a`

, `f (f a)`

, `f (f (f a))`

, etc. is `Free f a`

, so that `type ListBy Comp = Free`

. The type that is either `f a`

, `f (f a)`

, `f (f (f a))`

, etc. (at least one layer of `f`

) is `Free1 f a`

, so `type NonEmptyBy Comp = Free1`

.

*functor-combinators* provides functions like `toListBy :: t f f ~> ListBy t f`

to abstract over “converting” back and forth between `t f f a`

and linked list version `ListBy t f a`

(for example, between `Comp f f a`

and `Free f a`

).

### :+: / Sum

**Origin**:*GHC.Generics*(for`:+:`

) /*Data.Functor.Sum*(for`Sum`

)**Mixing Strategy**: “Either-or”: provide either case, and user has to handle both possibilities. Basically higher-order`Either`

.It can be useful for situations where you can validly use one or the other in your schema or functor. For example, if you are describing an HTTP request, we could have

`data GET a`

describing a GET request and`data POST a`

describing a POST request;`(GET :+: POST) a`

would be a functor that describes either a GET or POST request.The person who creates the

`f :+: g`

decides which one to give, and the person who consumes/interprets/runs the`f :+: g`

must provide a way of handling*both*`binterpret`

becomes analogous to`either`

from*Data.Either***Identity**`f :+: V1`

is equivalent to just`f`

, because you can never have a value of the right branch.**Monoids**`instance SemigroupIn (:+:) f instance MonoidIn (:+:) V1 f binterpret @(:+:) :: (g ~> f) -> (h ~> f) -> (g :+: h) a -> f a inL @(:+:) :: f ~> f :+: g inR @(:+:) :: g ~> f :+: g pureT @(:+:) :: V1 ~> h`

*All*haskell functors are monoids in`:+:`

. You can call`binterpret`

,`inL`

,`inR`

, etc. with anything.However, note that

`pureT`

is effectively impossible to call, because no values of type`V1 a`

exist.**List type**`Step`

is the result of an infinite application of`:+:`

to the same value:`type Step f = f :+: f :+: f :+: f :+: f :+: f :+: ... etc. -- actual implementation data Step f a = Step { stepPos :: Natural , stepVal :: f a }`

The correspondence is:

It’s not a particularly useful type, but it can be useful if you want to provide an

`f a`

alongside “which position” it is on the infinite list.

### :*: / Product

**Origin**:*GHC.Generics*(for`:*:`

) /*Data.Functor.Product*(for`Product`

)**Mixing Strategy**: “Both, separately”: provide values from*both*functors, and the user can choose which one they want to use. Basically a higher-order tuple.It can be useful for situations where your schema/functor must be

*specified*using*both*functors, but the*interpreter*can choose to use only one or the other (or both).**Identity**`instance Tensor (:*:) Proxy -- | Data type with only a single constructor and no information data Proxy a = Proxy`

`f :*: Proxy`

is equivalent to just`f`

, because the left hand side doesn’t add anything extra to the pair.**Monoids**`instance Alt f => SemigroupIn (:*:) f instance Plus f => MonoidIn (:*:) Proxy f binterpret @(:*:) :: Alt f => g ~> f -> h ~> f -> (g :*: h) ~> f inL @(:*:) :: Plus g => f ~> f :*: g inR @(:*:) :: Plus f => g ~> f :*: g pureT @(:*:) :: Plus h => Proxy ~> h`

`Alt`

, from*Data.Functor.Alt*in*semigroupoids*, can be thought of a “higher-kinded semigroup”: it’s like`Alternative`

, but with no`Applicative`

constraint and no identity:It is used to combine the results in both branches of the

`:*:`

.To introduce an “empty” branch, we need

`Plus`

(in*Data.Functor.Plus*), which is like a higher-kinded`Monoid`

, or`Alternative`

with no`Applicative`

:**List type**`ListF f a`

is a “list of`f a`

s”. It represents the possibility of having`Proxy`

(zero items),`x :: f a`

(one item),`x :*: y`

(two items),`x :*: y :*: z`

(three items), etc.It’s basically an ordered collection of

`f a`

s`:*:`

d with each other.It is useful if you want to define a schema where you can offer

*multiple*options for the`f a`

, and the interpreter/consumer can freely pick any one that they want to use.`NonEmptyF`

is the version of`ListF`

that has “at least one`f a`

”.See the information later on

`ListF`

alone (in the single-argument functor combinator section) for more information on usage and utility.

### Day

**Origin**:*Data.Functor.Day***Mixing Strategy**: “Both, together forever”: provide values from*both*functors, and the user*must*also*use*both.It can be useful for situations where your schema/functor must be

*specified*using*both*functors, and the user must also*use*both.Unlike for

`:*:`

, you always have to interpret*both*functor values in order to interpret a`Day`

. It’s a “full mixing”.The mechanism for this is interesting in and of itself. Looking at the definition of the data type:

We see that because

`x`

and`y`

are “hidden” from the external world, we can’t directly use them without applying the “joining” function`x -> y -> a`

. Due to how existential types work, we can’t get anything out of it that “contains”`x`

or`y`

. Because of this,*using*the joining function requires*both*`f x`

and`g y`

. If we only use`f x`

, we can only get, at best,`f (y -> a)`

; if we only use`g y`

, we can only get, at best,`g (x -> a)`

. In order to fully eliminate*both*existential variables, we need to get the`x`

and`y`

from*both*`f x`

and`g y`

, as if the two values held separate halves of the key.**Identity**`Day f Identity`

is equivalent to just`f`

, because`Identity`

adds no extra effects or structure.**Monoids**`instance Apply f => SemigroupIn Day f instance Applicative f => MonoidIn Day Identity f binterpret @Day :: Apply f => (g ~> f) -> (h ~> f) -> Day g h ~> f inL @Day :: Applicative g => f ~> Day f g inR @Day :: Applicative f => g ~> Day f g pureT @Day :: Applicative h => Identity ~> h`

`Apply`

, from*Data.Functor.Apply*in*semigroupoids*, is “`Applicative`

without`pure`

”; it only has`<*>`

(called`<.>`

).`pureT`

is essentially`pure :: Applicative h => a -> h a`

.**List type**`Ap f a`

is a bunch of`f x`

s`Day`

d with each other. It is either:`a`

(zero`f`

s)`f a`

(one`f`

)`Day f f a`

(two`f`

s)`Day f (Day f f) a`

(three`f`

s)- .. etc.

Like

`ListF`

this is very useful if you want your schema to provide a “bag” of`f a`

s and your interpreter*must use all of them*.For example, if we have a schema for a command line argument parser, each

`f`

may represent a command line option. To interpret it, we must look at*all*command line options.`Ap1`

is a version with “at least one”`f a`

.See the information later on

`Ap`

alone (in the single-argument functor combinator section) for more information on usage and utility.

### Comp

**Origin**:*Control.Monad.Freer.Church*. Note that an equivalent type is also found in*GHC.Generics*and*Data.Functor.Compose*, but they are incompatible with the`HBifunctor`

typeclass because they require the second input to have a`Functor`

instance.**Mixing Strategy**: “Both, together, sequentially” : provide values from*both*functors; the user must*use*both, and*in order*.It can be useful for situations where your schema/functor must be specified using both functors, and the user must

*use*both, but also enforcing that they must use both in the*given order*: that is, for a`Comp f g`

, they interpret`f`

*before*they interpret`g`

.Unlike for

`:*:`

, you always have to interpret*both*functor values. And, unlike for`Day`

, you must interpret both functor values*in that order*.**Identity**`Comp f Identity`

is equivalent to just`f`

, because`Identity`

adds no extra effects or structure.**Monoids**`instance Bind f => SemigroupIn Comp f instance Monad f => MonoidIn Comp Identity f binterpret @Comp :: Bind f => (g ~> f) -> (h ~> f) -> Comp g h ~> f inL @Comp :: Monad g => f ~> Comp f g inR @Comp :: Monad f => g ~> Comp f g pureT @Comp :: Monad h => Identity ~> h`

`Bind`

, from*[Data.Functor.Bind][]*in*semigroupoids*, is “`Monad`

without`return`

”; it only has`>>=`

(called`>>-`

).Somewhat serendipitously, the constraint associated with monoids in

`Comp`

is none other than the infamous`Monad`

.This might sound familiar to your ears — it’s the realization of the joke that “monads are monoids in the category of (endo)functors”. The idea is that we can make a tensor like

`Comp`

over functors, and that “monoids in” that tensor correspond exactly to`Monad`

instances. A part of the joke that we can now also see is that monads aren’t the*only*monoids in the category of endofunctors: they’re just the ones that you get when you tensor over`Comp`

. But we see now that if you use`Day`

as your tensor, then “monoids in the category of functors over`Day`

” are actually`Applicative`

instances! And that the monoids over`:*:`

are`Alt`

instances, etc.Theory aside, hopefully this insight also gives you some insight on the nature of

`Monad`

as an abstraction: it’s a way to “interpret” in and out of`Comp`

, which enforces an ordering in interpretation :)**List type**`Free f a`

is a bunch of`f x`

s composed with each other. It is either:`a`

(zero`f`

s)`f a`

(one`f`

)`f (f a)`

(two`f`

s)`f (f (f a))`

(three`f`

s)- .. etc.

`Free`

is very useful because it allows you to specify that your schema can have many`f`

s, sequenced one after the other, in which the*choice*of “the next`f`

” is allowed to depend on the*result*of “the previous`f`

”.For example, in an interactive “wizard” sort of schema, we can have a functor representing a dialog box with its result type:

We can then represent our wizard using

`Free Dialog a`

— an ordered sequence of dialog boxes, where the choice of the next box can depend on result of the previous box.`Free1`

is a version with “at least one”`f a`

.See the information later on

`Free`

alone (in the single-argument functor combinator section) for more information on usage and utility.

**Aside**

Let us pause for a brief aside to compare and contrast the hierarchy of the above functor combinators, as there is an interesting progression we can draw from them.

`:+:`

: Provide either, be ready for both.`:*:`

: Provide both, be ready for either.`Day`

: Provide both, be ready for both.`Comp`

: Provide both, be ready for both (in order).

### These1

**Origin**:*Data.Functor.These*.**Mixing Strategy**: “Either-or, or both”: provide either (or both) cases, and user has to handle both possibilities. An “inclusive either”This can be useful for situations where your schema/functor can be specified using one functor or another, or even both. See description on

`:+:`

for examples.The person who creates the

`These1 f g`

decides which one to give, and the person who consumes/interprets/runs the`f :+: g`

must provide a way of handling*both*situations.You can also pattern match on the

`These1`

directly to be more explicit with how you handle each of the tree cases.**Identity**`These1 f V1`

is equivalent to just`f`

, because it means the`That1`

and`These1`

branches will be impossible to construct, and you are left with only the`This1`

branch.**Monoids**`instance Alt f => SemigroupIn These1 f instance Alt f => MonoidIn These1 V1 f binterpret @These :: Alt f => (g ~> f) -> (h ~> f) -> These g h ~> f inL @These1 :: Alt g => f ~> Comp f g inR @These1 :: Alt f => g ~> Comp f g pureT @These1 :: Alt h => V1 ~> h`

You need at least

`Alt`

to be able to interpret out of a`These1`

, because you need to be able to handle the case where you have*both*`f`

and`g`

, and need to combine the result. However, you never need a full`Plus`

because we always have at least one value to use.**List type**`Steps`

, the list type, is the result of an infinite application of`These1`

to the same value:`type Steps f = f `These1` f `These1` f `These1` f `These1` ... etc. -- actual implementation newtype Steps f a = Steps (NEMap Natural (f a)) -- NEMap is a non-empty Map`

It essentially represents an infinite

*sparse*array of`f a`

s, where an`f a`

might exist at many different positions, with gaps here and there. There is always at least*one*`f a`

.Like

`Step`

, it’s not particularly useful, but it can be used in situations where you want a giant infinite sparse array of`f a`

s, each at a given position, with many gaps between them.I’ve skipped over the the “non-empty” version, which is

`ComposeT Flagged Steps`

; it requires an extra boolean “flag” because of some of the quirks of nonemptiness. I feel it is even less useful than`Steps`

.

### LeftF / RightF

**Origin**:*Data.HBifunctor***Mixing Strategy**: “Ignore the left” / “ignore the right”.You can think of

`LeftF`

as “`:+:`

without the Right case,`R1`

”, or`RightF`

as “`:+:`

without the Left case,`L1`

”.`RightF`

can be considered a higher-order version of*Tagged*, which “tags” a value with some type-level information.This can be useful if you want the second (or first) argument to be ignored, and only be used maybe at the type level.

For example,

`RightF IgnoreMe MyFunctor`

is equivalent to just`MyFunctor`

, but you might want to use`IgnoreMe`

as a phantom type to help limit what values can be used for what functions.**Identity**Unlike the previous functor combinators, these three are only

`Associative`

, not`Tensor`

: this is because there is no functor`i`

such that`LeftF i g`

is equal to`g`

, for all`g`

, and no functor`i`

such that`RightF f i`

is equal to`f`

, for all`f`

.**Constraints**Interpreting out of either of these is unconstrained, and can be done in any context.

**List type**For

`LeftF`

, the non-empty list type is`Flagged`

, which is the`f a`

tupled with a`Bool`

. See the information on`Flagged`

for more details. This can be useful as a type that marks if an`f`

is made with`inject`

/`pure`

and is “pure” (`False`

), or “tainted” (`True`

). The*provider*of a`Flagged`

can specify “pure or tainted”, and the*interpreter*can make a decision based on that tag.For

`RightF`

, the non-empty list type is`Step`

. See`Step`

and the information on`:+:`

for more details. This can be useful for having a value of`f a`

at “some point”, indexed by a`Natural`

.

## Single-Argument

Unary functor combinators usually directly “enhance” a functor with extra capabilities — usually in the form of a typeclass instance, or extra data fields/constructors.

All of these can be “lifted into” with any constraint on `f`

.

`Inject`

seems very similar to `MonadTrans`

’s `lift`

; the difference is that `inject`

must be *natural* on `f`

: it can assume nothing about the structure of `f`

, and must work universally the same. `MonadTrans`

, in contrast, requires `Monad f`

.

Each one can also be “interpreted to” certain functors `f`

:

An important law is that:

This means that if we inject and immediately interpret out of, we should never *lose* any information in `f`

. All of the original structure in `f`

must stay intact: functor combinators only ever *add* structure.

### Coyoneda

**Origin**:*Data.Functor.Coyoneda***Enhancement**: The ability to map over the parameter; it’s the free`Functor`

.Can be useful if

`f`

is created using a`GADT`

that cannot be given a`Functor`

instance.For example, here is an indexed type that represents the type of a “form element”, where the type parameter represents the output result of the form element.

`data FormElem :: Type -> Type where FInput :: FormElem String FTextbox :: FormElem Text FCheckbox :: FormElem Bool FNumber :: FormElem Int`

Then

`Coyoneda FormElem`

has a`Functor`

instance. We can now fmap over the result type of the form element; for example,`fmap :: (a -> b) -> Coyoneda FormElem a -> Coyoneda FormElem b`

takes a form element whose result is an`a`

and returns a form element whose result is a`b`

.**Interpret**`instance Functor f => Interpret Coyoneda f interpret @Coyoneda :: Functor f => g ~> f -> Coyoneda g ~> f`

Interpreting out of a

`Coyoneda f`

requires the target context to itself be`Functor`

. Usually, the context is an`Applicative`

or`Monad`

, so this is typically always satisfied.For example, if we want to “run” a

`Coyoneda FormElem`

in`IO`

(maybe as an interactive CLI form), this would be`interpret :: (forall x. FormElem x -> IO x) -> Coyoneda FormElem a -> IO a`

.

### ListF / NonEmptyF

**Origin**:*Control.Applicative.ListF***Enhancement**: The ability to offer multiple options for the interpreter to pick from;`ListF`

is the free`Plus`

, and`NonEmptyF`

is the free`Alt`

.`data ListF f a = ListF { runListF :: [f a] } data NonEmptyF f a = NonEmptyF { runNonEmptyF :: NonEmpty (f a) }`

Can be useful if you want to provide the ability when you

*define*your schema to provide multiple`f a`

s that the*interpreter*/consumer can freely pick from.For example, for a schema specifying a form, you might have multiple ways to enter a name. If you had a

`Name`

schema`data Name a`

, then you can represent “many different potential name inputs” schema as`ListF Name a`

.Because this has a

`Plus`

instance, you can use`(<!>) :: ListF f a -> ListF f a -> ListF f a`

to combine multiple option sets, and`zero :: ListF f a`

to provide the “choice that always fails/is unusuable”.`NonEmptyF`

is a variety of`ListF`

where you always have “at least one`f a`

”. Can be useful if you want to ensure, for your interpreter’s sake, that you always have at least one`f a`

option to pick from. For example,`NonEmptyF Name a`

will always have at least*one*name schema.This is essentially

`f`

`:*:`

d with itself multiple times;`ListF`

is the linked list list made by`:*:`

, and`NonEmptyF`

is the non-empty linked list made by`:*:`

.**Interpret**`instance Plus f => Interpret ListF f instance Alt f => Interpret NonEmptyF f interpret @ListF :: Plus f => g ~> f -> ListF g ~> f interpret @NonEmptyF :: Alt f => g ~> f -> NonEmptyF g ~> f`

Interpreting out of a

`ListF f`

requires the target context to be`Plus`

, and interpreting out of a`NonEmptyF f`

requires`Alt`

(because you will never have the empty case). However, you always have the option to directly pattern match on the list and pick an item you want directly, which requires no constraint.

### Ap / Ap1

**Enhancement**: The ability to provide multiple`f`

s that the interpreter*must*consume*all*of;`Ap`

is the free`Applicative`

, and`Ap1`

is the free`Apply`

.While

`ListF`

may be considered “multiple options*offered*”,`Ap`

can be considered “multiple actions all*required*”. The interpreter must consume/interpret*all*of the multiple`f`

s in order to interpret an`Ap`

.For example, for a form schema, you might want to have multiple form elements. If a single form element is

`data FormElem a`

, then you can make a multi-form schema with`Ap FormElem a`

. The consumer of the form schema must handle*every*`FormElem`

provided.Note that ordering is not enforced: while the consumer must handle each

`f`

eventually, they are free to handle it in whatever order they desire. In fact, they could even all be handled in parallel. See`Free`

for a version where ordering is enforced.Because this has an

`Applicative`

instance, you can use`(<*>) :: Ap f (a -> b) -> Ap f a -> Ap f b`

to sequence multiple`Ap f`

s together, and`pure :: a -> Ap f a`

to produce a “no-op”`Ap`

without any`f`

s.`Ap`

has some utility over`Free`

in that you can pattern match on the constructors directly and look at each individual sequenced`f a`

, for static analysis, before anything is ever run or interpreted.*Structurally*,`Ap`

is built like a linked list of`f x`

s, which each link being existentially bound together:`data Ap :: (Type -> Type) -> Type -> Type where Pure :: a -> Ap f a Ap :: f a -> Ap f (a -> b) -> Ap f b`

`Pure`

is like “nil”, and`Ap`

is like “cons”:The existential type in the

`Ap`

branch plays the same role that it does in the definition of`Day`

(see the description of`Day`

for more information).`Ap1`

is a variety of`Ap`

where you always have to have “at least one`f`

”. Can be useful if you want to ensure, for example, that your form has at least one element.Note that this is essentially

`f`

`Day`

d with itself multiple times;`Ap`

is the linked list made by`Day`

and`Ap1`

is the non-empty linked list made by`Day`

.**Interpret**`instance Applicative f => Interpret Ap f instance Apply f => Interpret Ap1 f interpret @Ap :: Applicative f => g ~> f -> Ap g ~> f interpret @Ap1 :: Apply f => g ~> f -> Ap1 g ~> f`

Interpreting out of an

`Ap f`

requires the target context to be`Applicative`

, and interpreting out of a`Ap1 f`

requires`Apply`

(because you will never need the pure case).

### Alt

**Origin**:*Control.Alternative.Free***Enhancement**: A combination of both`ListF`

and`Ap`

: provide a choice (`ListF`

-style) of sequences (`Ap`

-style) of choices of sequences of choices ….; it’s the free`Alternative`

.This type imbues

`f`

with both sequential “must use both” operations (via`<*>`

) and choice-like “can use either” operations (via`<|>`

).It can be useful for implementing parser schemas, which often involve both sequential and choice-like combinations. If

`f`

is a primitive parsing unit, then`Alt f`

represents a non-deterministic parser of a bunch of`f`

s one after the other, with multiple possible results. I wrote an entire article on the usage of this combinator alone to implement a version of regular expressions.**Interpret**Interpreting out of an

`Alt f`

requires the target context to be`Alternative`

— it uses`<*>`

for sequencing, and`<|>`

for choice.

### Free / Free1

**Origin**:*Control.Monad.Freer.Church*, which is a variant of*Control.Monad.Free*that is compatible with`HFunctor`

.**Enhancement**: The ability to provide multiple`f`

s that the interpreter must consume*in order*, sequentially — the free`Monad`

.Contrast with

`Ap`

, which also sequences multiple`f`

s together, but without any enforced order. It does this by*hiding*the “next`f a`

” until the previous`f a`

has already been interpreted.Perhaps more importantly, you can sequence

`f`

s in a way where the*choice of the next*is allowed to depend on the`f`

*result of the previous*.`f`

For example, in an interactive “wizard” sort of schema, we can create a functor to represent a dialog box with its result type:

We can then construct a type for a wizard:

`Wizard`

is now an ordered sequence of dialog boxes, where the choice of the next box can depend on result of the previous box. Contrast to`Ap Dialog`

, where the choice of all dialog boxes must be made in advanced, up-front, before reading any input from the user.In having this, however, we loose the ability to be able to inspect each

`f a`

before interpreting anything.Because this has a

`Monad`

instance, you can use`(<*>) :: Free f (a -> b) -> Free f a -> Free f b`

and`(>>=) :: Free f a -> (a -> Free f b) -> Free f b)`

to sequence multiple`Free f`

s together, and`pure :: a -> Free f a`

to produce a “no-op”`Free`

without any`f`

s.`Free1`

is a variety of`Free1`

where you always have to have “at least one`f`

”. Can be useful if you want to ensure, for example, that your wizard has at least one dialog box.Note that this is essentially

`f`

`Comp`

d with itself multiple times;`Free`

is the linked list made by`Comp`

and`Free1`

is the non-empty linked list made by`Comp`

.**Interpret**`instance Monad f => Interpret Free f instance Bind f => Interpret Free1 f interpret @Free :: Monad f => g ~> f -> Free g ~> f interpret @Free1 :: Bind f => g ~> f -> Free1 g ~> f`

Interpreting out of a

`Free f`

requires the target context to be`Monad`

, and interpreting out of a`Free1 f`

requires`Bind`

(because you will never need the pure case).

### Lift / MaybeApply

**Origin**:*Control.Applicative.Lift*/*Data.Functor.Apply*(the same type)**Enhancement**: Make`f`

“optional” in the schema in a way that the interpreter can still work with as if the`f`

was still there; it’s the free`Pointed`

.`data Lift f a = Pure a | Other (f a) newtype MaybeApply f a = MaybeApply { runMaybeApply :: Either a (f a) } -- ^ same type, from semigroupoids`

Can be useful so that an

`f a`

is*optional*for the schema definition, but in a way where the consumer can still continue from it as if they*had*the`f`

.It can be used, for example, to turn an required parameter

`Param a`

into an optional parameter`Lift Param a`

.Contrast this to

`MaybeF`

: this allows the interpreter to still “continue on” as normal even if the`f`

is not there. However,`MaybeF`

forces the interpreter to abort if the`f`

is not there.This can be thought of as

`Identity :+: f`

.**Interpret**Interpreting out of a

`Lift f`

requires the target context to be`Pointed`

, from*Data.Pointed*— it uses`point :: Pointed f => a -> f a`

to handle the case where the`f`

is not there.

### MaybeF

**Origin**:*Control.Applicative.ListF***Enhancement**: Make`f`

“optional” in the schema in a way that the interpreter*must fail*if the`f`

is not present.Can be useful so that an

`f a`

is*optional*for the schema definition; if the`f`

is not present, the consumer must abort the current branch, or find some other external way to continue onwards.Contrast this to

`Lift`

, which is an “optional”`f`

that the consumer may continue on from.**Interpret**Interpreting out of a

`Lift f`

requires the target context to be`Plus`

— it uses`zero :: f a`

to handle the case where the`f`

is not there. Note that this is actually “over-constrained”: we really only need`zero`

, and not all of`Plus`

(which includes`<!>`

). However, there is no common typeclass in Haskell that provides this, so this is the most pragmatic choice.

### EnvT

**Origin**:*Control.Comonad.Trans.Env***Enhancement**: Provide extra (monoidal) data alongside`f a`

that the interpreter can access. Basically tuples extra`e`

alongside the`f a`

.You can use this to basically tuple some extra data alongside an

`f a`

. It can be useful if you want to provide extra information that isn’t inside the`f`

for the interpreter use for interpretation.When using

`inject :: Monoid e => f a -> EnvT e f a`

, it uses`mempty`

as the initial`e`

value.One of my personal favorite uses of

`EnvT`

is the*flare*purescript library, which uses the`e`

as the observed HTML of a form, and the`f a`

as an active way to get information from a form interactively.`inject`

is used to insert an active form element without caring about its HTML representation, and`interpret`

would “run” the active elements to get the results.This type exists specialized a few times here, as well:

`Step`

is`EnvT (Sum Natural)`

`Flagged`

is`EnvT Any`

**Interpret**Interpreting out of

`EnvT e`

requires no constraints.

### MapF / NEMapF

**Origin**:*Control.Applicative.ListF***Enhancement**: Contain multiple`f a`

s, each indexed at a specific*key*.`newtype MapF k f a = MapF { runMapF :: Map k (f a) } newtype NEMapF k f a = NEMapF { runMapF :: NEMap k (f a) }`

This is very similar in functionality to

`ListF`

and`NonEmptyF`

, except instead of “positional” location, each`f a`

exists at a given index.`NEMapF k`

is the “non-empty” variant. You can think of this as a`ListF`

plus`EnvT`

: it’s a “container” of multiple`f a`

s, but each one exists with a given “tag” index`k`

.In usage, like for

`ListF`

, the*definer*provides multiple “labeled”`f a`

s, and the*interpreter*can choose to interpret some or all of them, with access to each labeled.`inject`

creates a singleton`Map`

at key`mempty`

.This is very useful in schemas that have sub-schemas indexed at specific keys. For example, in a command line argument parser, if we have a functor that represents a single command:

We can immediately promote it to be a functor representing

*multiple possible*named commands, each at a given string:So we can implement “git push” and “git pull” using:

`push :: Command Action pull :: Command Action gitCommands :: Commands Action gitCOmmands = MapF . M.fromList $ [ ("push", push) , ("pull", pull) ]`

This is also useful for specifying things like routes in a server.

This type exists specialized as

`Steps`

, which is`NEMapF (Sum Natural)`

.**Interpret**`instance Plus f => Interpret (MapF k) f instance Alt f => Interpret (NEMap k) f interpret @(MapF k) :: Plus f => g ~> f -> MapF g ~> f interpret @(NEMapF k) :: Alt f => g ~> f -> NEMapF g ~> f`

Interpreting out of a

`MapF k f`

requires the target context to be`Plus`

, and interpreting out of a`NEMapF k f`

requires`Alt`

(because you will never have the empty case). However, you can directly*look up*into the`Map`

and pick an item you want directly, which requires no constraint.

### ReaderT

**Origin**:*Control.Monad.Trans.Reader***Enhancement**: Provide each`f a`

with access to some “environment”`r`

.`ReaderT r`

is often used to model some form of dependency injection: it allows you to work “assuming” you had an`r`

; later, when you*run*it, you provide the`r`

. It delays the evaluation of your final result until you provide the missing`r`

.Another way of looking at it is that it makes your entire functor have values that are

*parameterized*with an`r`

.For example, if you have a form data type:

you can now make a form data type that is parameterized by the current server hostname:

The actual structure of your

`FormElem`

is deferred until you provide the`HostName`

.Note that, unlike

`ReaderT`

, most monad transformers from*transformers*are actually*not*valid functor combinators under our perspective here, because most of them are not*natural*on`f`

: they require`Functor f`

, at least, to implement`inject`

or`hmap`

.**Interpret**`instance MonadReader r f => Interpret (ReaderT r) f interpret @(ReaderT r) :: MonadReader r f => g ~> f -> ReaderT r g ~> f`

Interpreting out of a

`ReaderT r`

requires requires the target context to be`MonadReader r`

, which means it must have access to`ask :: MonadReader r f => f r`

.In a way,

`ReaderT r`

is the “free” instance of`MonadReader r`

.

### Step

**Origin**:*Control.Applicative.Step***Enhancement**: Tuples the`f a`

with an extra natural number index.This is essentially a specialized

`EnvT`

: it’s`EnvT (Sum Natural)`

.This is a useful type because it can be seen as equivalent to

`f :+: f :+: f :+: f :+: f ...`

forever: it’s an`f`

, but at some index. In*Control.Applicative.Step*, we have specialized functions`stepUp`

and`stepDown`

, which allows you to “match” on the “first”`f`

in that infinite chain; it will increment and decrement the index relatively to make this work properly.**Interpret**Interpreting out of

`Step`

requires no constraints; we just drop the`Natural`

data.

### Steps

**Origin**:*Control.Applicative.Step***Enhancement**: The ability to offer multiple*indexed*options for the interpreter to pick from. Like`NonEmptyF`

, except with each`f a`

existing at an indexed position that the consumer/interpreter can look up or access.This is like a mix between

`NonEmptyF`

and`Step`

: multiple`f a`

options (at least one) for the consumer/interpreter to pick from. Unlike`NonEmptyF`

, each`f a`

exists at an “index” — there might be one at 0, one at 5, one at 100, etc.Another way of looking at this is like an infinite

*sparse array*of`f a`

s: it’s an inifinitely large collection where each spot may potentially have an`f a`

.Useful for “provide options that the consumer can pick from, index, or access”, like

`ListF`

/`NonEmptyF`

.This type can be seen as an infinite

`f `These1` f `These1` f `These1` f ...`

, and along these lines,`stepsDown`

and`stepsUp`

exist inside*Control.Applicative.Step*analogous to`stepUp`

and`stepDown`

to treat a`Steps`

in this manner.**Interpret**Interpreting out of

`Steps`

requires an`Alt`

to combine different possibilities. It does not require a full`Plus`

constraint because we never need`zero`

: a`Steps f a`

always has at least one`f a`

.

### Flagged

**Origin**:*Control.Applicative.Step***Enhancement**: The ability to “tag” a functor value with a`True`

/`False`

boolean value.This is essentially a specialized

`EnvT`

: it’s`EnvT Any`

.If created with

`inject`

or`pure`

, it adds the flag`False`

. This is helpful for helping indicate if the value was created using a “pure” method like`inject`

or`pure`

, or an “impure” method (any other method, including direct construction).**Interpret**Interpreting out of

`Flagged`

requires no constraints; we just drop the boolean flag.

### Final

**Origin**:*Data.HFunctor.Final***Enhancement**:`Final c`

will lift`f`

into a free structure of any typeclass`c`

; it will give it all of the actions/API of a typeclass for “free”.`Final c f`

is the “free`c`

” over`f`

.In a way, this is the “ultimate free structure”: it can fully replace all other free structures of typeclasses of kind

`Type -> Type`

. For example:`Coyoneda ~ Final Functor ListF ~ Final Plus NonEmptyF ~ Final Alt Ap ~ Final Applicative Ap1 ~ Final Apply Free ~ Final Monad Free1 ~ Final Bind Lift ~ Final Pointed IdentityT ~ Final Unconstrained`

All of these are connections are witnessed by instances of the typeclass

`FreeOf`

in*Data.HFunctor.Final*.In fact,

`Final c`

is often more performant for many operations than the actual concrete free structures.The main downside is that you cannot directly pattern match on the structure of a

`Final c`

the same way you can pattern match on, say,`Ap`

or`ListF`

. However, you can get often around this by using`Final Plus`

for most of your operations, and then`interpret inject`

-ing it into`ListF`

when you want to actually pattern match.You can also think of this as the “ultimate

`Interpret`

”, because with`inject`

you can push`f`

into`Final c f`

, and with`interpret`

you only ever need the`c`

constraint to “run”/interpret this.So, next time you want to give an

`f`

the ability to`<*>`

and`pure`

, you can throw it into`Final Applicative`

:`f`

now gets “sequencing” abilities, and is equivalent to`Ap f`

.If you want the API of a given typeclass

`c`

, you can inject`f`

into`Final c`

, and you get the API of that typeclass for free on`f`

.**Constraint**Interpreting out of a

`Final c`

requires`c`

, since that is the extra context that`f`

is lifted into.

### Chain / Chain1

**Origin**:*Data.HFunctor.Chain***Enhancement**:`Chain t`

will lift`f`

into a linked list of`f`

s chained by`t`

.`-- i is intended to be the identity of t data Chain t i f a = Done (i a) | More (t f (Chain t i f a))`

For example, for

`:*:`

,`Chain (:*:) Proxy f`

is equivalent to one of:`Proxy <=> Done Proxy <=> ListF [] x <=> More (x :*: Done Proxy) <=> ListF [x] x :*: y <=> More (x :*: More (y :*: Done Proxy)) <=> ListF [x,y] -- etc.`

For

`:+:`

,`Chain (:+:) V1 f`

is equivalent to one of:`L1 x <=> More (L1 x) <=> Step 0 x R1 (L1 y) <=> More (R1 (More (L1 y))) <=> Step 1 y R1 (R1 (L1 z)) <=> More (R1 (More (R1 (More (L1 z))))) <=> Step 2 z -- etc.`

This is useful because it provides a nice uniform way to work with all “linked list over tensors”. That’s because the following types are all isomorphic:

`ListF ~ Chain (:*:) Proxy Ap ~ Chain Day Identity Free ~ Chain Comp Identity Step ~ Chain (:+:) Void Steps ~ Chain These1 Void`

This isomorphism is witnessed by

`unroll`

(turn into the`Chain`

) and`reroll`

(convert back from the`Chain`

) in*Data.HFunctor.Chain*.We can “fold down” a

`Chain t (I t) f a`

into an`f a`

, if`t`

is`Monoidal`

, using`interpret id`

. In fact, this ability could be used as a fundamental property of monoidal nature.We also have a “non-empty” version,

`Chain1`

, for non-empty linked lists over tensors:`NonEmptyF ~ Chain1 (:*:) Ap1 ~ Chain1 Day Free1 ~ Chain1 Comp Step ~ Chain1 (:+:) Steps ~ Chain1 These1 EnvT Any ~ Chain1 LeftF Step ~ Chain1 RightF`

We can “fold down” a

`Chain1 t f a`

into an`f a`

, if`t`

is`Semigroupoidal`

, using`interpret id`

. In fact, this ability could be used as a fundamental property of semigroupoidal nature.Using

`ListF`

,`Ap`

,`Free`

,`Step`

,`Steps`

, etc. can sometimes feel very different, but with`Chain`

you get a uniform interface to pattern match on (and construct) all of them in the same way.Using

`NonEmptyF`

,`Ap1`

,`Free1`

,`Step`

,`Flagged`

, etc. can sometimes feel very different, but with`Chain1`

you get a uniform interface to pattern match on (and construct) all of them in the same way.Universally, we can concatenate linked chains, with:

`appendChain :: Tensor t i => t (Chain t i f) (Chain t i f) ~> Chain t i f appendChain1 :: Associative t => t (Chain1 t f) (Chain1 t f) ~> Chain1 t f`

These operations are associative, and this property is gained from the tensor nature of

`t`

.The construction of

`Chain`

is inspired by Oleg Grenrus’s blog post, and the construction of`Chain1`

is inspired by implementations of finite automata and iteratees.**Interpret**`instance MonoidIn t i f => Interpret (Chain t i) f instance SemigroupIn t f => Interpret (Chain1 t ) f interpret @(Chain t i) :: MonoidIn t i f => g ~> f -> Chain t i g ~> f interpret @(Chain1 t) :: SemigroupIn t f => g ~> f -> Chain1 t g ~> f`

Interpreting out of a

`Chain`

requires only that`f`

is a monoid in`t`

. Interpreting out of a`Chain1`

requires only that`f`

is a semigroup in`t`

.For example, we have:

`instance Plus f => Interpret (Chain (:*:) Proxy) f instance Alt f => Interpret (Chain1 (:*:) ) f interpret @(Chain (:*:) Proxy) :: Plus f => g ~> f -> Chain (:*:) Proxy g ~> f interpret @(Chain1 (:*:)) :: Alt f => g ~> f -> Chain1 (:*:) f ~> f instance Applicative f => Interpret (Chain Day Identity) f instance Apply f => Interpret (Chain1 Day ) f instance Monad f => Interpret (Chain Comp Identity) f instance Bind f => Interpret (Chain1 Comp ) f`

### IdentityT

**Origin**:*Data.Functor.Identity***Enhancement**: None whatsoever; it adds no extra structure to`f`

, and`IdentityT f`

is the same as`f`

; it’s the “free`Unconstrained`

”This isn’t too useful on its own, but it can be useful to give to the functor combinator combinators as a no-op functor combinator. It can also be used to signify “no structure”, or as a placeholder until you figure out what sort of structure you want to have.

In that sense, it can be thought of as a “

`ListF`

with always one item”, a “`MaybeF`

that’s always`Just`

”’, an “`Ap`

with always one sequenced item”, a “`Free`

with always exactly one layer of effects”, etc.**Constraint**Interpreting out of

`IdentityT`

requires no constraints — it basically does nothing.

### ProxyF / ConstF

**Origin**:*Data.HFunctor***Enhancement**: “Black holes” — they completely forget all the structure of`f`

, and are impossible to`interpret`

out of.`Impossible`

".`ProxyF`

is essentially`ConstF ()`

.These are both valid functor combinators in that you can inject into them, and

`interpret id . inject == id`

is*technically*true (the best kind of true).You can use them if you want your schema to be impossible to interpret, as a placeholder or to signify that one branch is uninterpretable. In this sense, this is like a “

`ListF`

that is always empty” or a “`MaybeF`

that is always`Nothing`

”.Because of this, they aren’t too useful on their own — they’re more useful in the context of swapping out and combining or manipulating with other functor combinators or using with functor combinator combinators.

**Interpret**You’re not going to have any luck here — you cannot interpret out of these, unfortunately!

## Combinator Combinators

There exist higher-order functor combinator combinators that take functor combinators and return new ones, too. We can talk about a uniform interface for them, but they aren’t very common, so it is probably not worth the extra abstraction.

### ComposeT

**Origin**:*Control.Monad.Trans.Compose***Enhancement**: Compose enhancements from two different functor combinatorsCan be useful if you want to layer or nest functor combinators to get both enhancements as a

*single*functor combinator*.Usually really only useful in the context of other abstractions that expect functor combinators, since this is the best way to turn two functor combinators into a third one.

**Interpret**`instance (Interpret s f, Interpret t f) => Interpret (ComposeT s t) f interpret @(ComposeT s t) :: (Interpret s f, Interpret t f) => g ~> f -> ComposeT s t g ~> f`

Interpreting out of these requires the constraints on

*both*layers.

### HLift

**Origin**:*Data.HFunctor***Enhancement**:`HLift t f`

lets`f`

exist either unchanged, or with the structure of`t`

.Can be useful if you want to “conditionally enhance”

`f`

. Either`f`

can be enhanced by`t`

, or it can exist in its pure “newly-injected” form.If

`t`

is`Identity`

, we get`EnvT Any`

, or`f :+: f`

: the “pure or impure” combinator.**Interpret**`instance Interpret t f => Interpret (HLift t) f interpret @(HLift t) :: Interpret t f => g ~> f -> HLift t g ~> f`

Interpreting out of these requires the constraint on

`t`

, to handle the`HOther`

case.

### HFree

**Origin**:*Data.HFunctor***Enhancement**:`HFree t f`

lets`f`

exist either unchanged, or with multiple nested enhancements by`t`

.It is related to

`HLift`

, but lets you lift over arbitrary many compositions of`t`

, enhancing`f`

multiple times. This essentially creates a “tree” of`t`

branches.One particularly useful functor combinator to use is

`MapF`

. In our earlier examples, if we haveto represent the structure of a single command line argument parser, we can use

to represent

*multiple*potential named commands, each under a different`String`

argument. With`HFree`

, we can also use:to represent

*nested*named commands, where each nested sub-command is descended on by a`String`

key.For another example,

`HFree IdentityT`

is essentially`Step`

.**Interpret**`instance Interpret t f => Interpret (HFree t) f interpret @(HFree t) :: Interpre t f => g ~> f -> HFree t g ~> f`

Interpreting out of these requires the constraint on

`t`

, to handle the`HJoin`

case.However, it is probably usually more useful to directly pattern match on

`HReturn`

and`HJoin`

and handle the recursion explicitly.Alternatively, we can also define a recursive folding function (provided in

*Data.HFunctor*) to recursively fold down each branch:This can be useful because it allows you to distinguish between the different branches, and also requires no constraint on

`g`

.Applied to the

`CommandTree`

example, this becomes:

## Closing Comments

As I discover more interesting or useful functor combinators (or as the abstractions in *functor-combinators* change), I will continue to update this post. And, in the upcoming weeks and months I plan to present specific programs I have written (and simple examples of usage) that will help show this design pattern in use within a real program.

For now, I hope you can appreciate this as a reference to help guide your exploration of unique “a la carte” (yet not fixed-point-centric) approach to building your programs! You can jump right into using these tools to build your program *today* by importing *Data.Functor.Combinator* or wherever they can be found.

I’d be excited to hear about what programs you are able to write, so please do let me know! You can leave a comment, find me on [twitter at @mstk]twitter, or find me on freenode irc idling on *#haskell* as *jle`* if you want to share, or have any questions.

## Special Thanks

I am very humbled to be supported by an amazing community, who make it possible for me to devote time to researching and writing these posts. Very special thanks to my supporter at the “Amazing” level on patreon, Josh Vera! :)

Also a special thanks to Koz Ross, who helped proofread this post as a draft.

On the surface, this functor combinator design pattern might look like it fills a similar space to effects systems and libraries like

*mtl*,*polysemy*,*freer-simple*, or*fused-effects*. However, this design pattern actually exists on a different level.Functor combinator design patterns can be used to help build the

*structure*of the*data types*and schemas that define your program/DSL. Once you build these nice structures, you then*interpret*them into some target context. This “target context” is the realm that libraries like*mtl*and*polysemy*can fill; functor combinators serve to help you define a structure for your program*before*you interpret it into whatever Applicative or Monad or effects system you end up using.↩︎