# Introduction to Singletons (Part 2)

by Justin Le ♦

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Welcome back to our journey through the singleton design pattern and the great *singletons* library!

This post is a direct continuation of Part 1, so be sure to check that out first if you haven’t already! If you hare just jumping in now, I suggest taking some time to to through the exercises if you haven’t already!

Again, code is built on *GHC 8.6.1* with the *nightly-2018-09-29* snapshot (so, *singletons-2.5*). However, unless noted, all of the code should still work with *GHC 8.4* and *singletons-2.4*. All of the code is also available here, and you can drop into a ghci session with all of the bindings in scope by executing the file:

## Review

Let’s return to our `Door`

type:

```
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/singletons/Door2.hs#L23-L29
$(singletons [d|
data DoorState = Opened | Closed | Locked
deriving (Show, Eq)
|])
data Door :: DoorState -> Type where
UnsafeMkDoor :: { doorMaterial :: String } -> Door s
```

First, this derives the *type* `DoorState`

with the values `Opened`

, `Closed`

, and `Locked`

, and also the *kind* `DoorState`

with the *types* `'Opened`

, `'Closed`

, and `'Locked`

. We then also derive the singletons (and implicit-style typeclass instances, reflectors, etc.) with the template haskell.

Then, there’s `Door`

. `Door`

is great! It is an *indexed data type* (indexed by a type of kind `DoorState`

) in that picking a different type variable gives a different “type” of Door:

`Door 'Opened`

is a type that represents the type of an opened door`Door 'Closed`

is a*different*type that represents the type of a*closed*door`Door 'Locked`

is yet another (third) type that represents the type of a*locked*door.

So, really, when we define `Door s`

, we really are defining *three distinct* types^{1}.

This is great and all, but isn’t Haskell a language with static, compile-time types? Doesn’t that mean that we have to know if our doors are opened, closed, or locked at compile-time?

This is something we can foresee being a big issue. It’s easy enough to create a `Door s`

if you know `s`

at compile-time by just typing in a type annotation (`UnsafeMkDoor "Oak" :: Door 'Opened`

) or by using a monomorphic constructor (`mkDoor SOpened "Oak"`

). But what if we *don’t* know `s`

at compile-time?

To learn how to do this, we first need to learn how to *not care*.

## Ditching the Phantom

Sometimes we don’t *actually* care about the state of the door in the *type* of the door. We don’t want `Door 'Opened`

and `Door 'Closed`

…we want a type to just represent a door, without the status in its type.

This might come about a bunch of different ways. Maybe you’re reading a `Door`

data from a serialization format, and you want to be able to parse *any* door (whatever door is serialized).

To learn how to not care, we can describe a type for a door that does *not* have its status in its type.

We have a couple of options here. First, we can create a new type `SomeDoor`

that is the same as `Door`

, except instead of keeping its status in its type, it keeps it as a runtime value:

```
data SomeDoor = MkSomeDoor
{ someDoorState :: DoorState
, someDoorMaterial :: String
}
-- or, in GADT syntax
data SomeDoor :: Type where
MkSomeDoor ::
{ someDoorState :: DoorState
, someDoorMaterial :: String
} -> SomeDoor
```

Note the similarity of `SomeDoor`

’s declaration to `Door`

’s declaration above. It’s mostly the same, except, instead of `DoorState`

being a type parameter, it is instead a runtime value inside `SomeDoor`

.

Now, this is actually a type that we *could* have been using this entire time, if we didn’t care about type safety. In the real world and in real applications, we actually might have written `SomeDoor`

*before* we ever thought about `Door`

with a phantom type. It’s definitely the more typical “standard” Haskell thing.

`SomeDoor`

is great. But because it’s a completely different type, we can’t re-use any of our `Door`

functions on this `SomeDoor`

. We potentially have to write the same function twice for both `Door`

and `SomeDoor`

, because they have different implementations.

### The Existential Datatype

However, there’s another path we can take. With the power of singletons, we can actually implement `SomeDoor`

*in terms of* `Door`

, using an **existential data type**:

```
-- using existential constructor syntax
data SomeDoor = forall s. MkSomeDoor (Sing s) (Door s)
-- or, using GADT syntax (preferred)
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/singletons/Door2.hs#L60-L61
data SomeDoor :: Type where
MkSomeDoor :: Sing s -> Door s -> SomeDoor
```

(Remember that `Sing s`

, when `s`

is a `DoorState`

, is a type “synonym” for our favorite door singleton `SDoorState s`

. We’re going to switch to using `Sing s`

instead of `SDoorState s`

for the rest of this series just to move into a more universal style where we treat the `Sing`

as basically syntactical noise)

`MkSomeDoor`

is a constructor for an existential data type, meaning that the data type “hides” a type variable `s`

. Note the type (`Sing s -> Door s -> SomeDoor`

) and how the result type (`SomeDoor`

) *forgets* the `s`

and hides all traces of it. Think of it like a type variable sponge – type variable goes in, but it’s absorbed opaquely into the result type.

Note the similarities between our original `SomeDoor`

and this one.

```
-- | Re-implementing door
data SomeDoor where
MkSomeDoor :: DoorState -> String -> SomeDoor
-- | Re-using Door, as an existential type
data SomeDoor where
MkSomeDoor :: Sing s -> Door s -> SomeDoor
-- ^ data Door s = UnsafeMkDoor String
```

Basically, our type before re-implements `Door`

. But the new one actually directly uses the original `Door s`

. This means we can *directly* re-use our `Door`

functions on `SomeDoor`

s, without needing to write completely new implementations.

In Haskell, existential data types are pretty nice, syntactically, to work with. Let’s write some basic functions to see. First, a function to “make” a `SomeDoor`

from a `Door`

:

```
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/singletons/Door2.hs#L63-L67
fromDoor :: Sing s -> Door s -> SomeDoor
fromDoor = MkSomeDoor
fromDoor_ :: SingI s => Door s -> SomeDoor
fromDoor_ = fromDoor sing
```

So that’s how we *make* one…how do we *use* it? Let’s port our `Door`

functions to `SomeDoor`

, by re-using our pre-existing functions whenever we can, and *pattern matching* on `MkSomeDoor`

:

```
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/singletons/Door2.hs#L69-L76
closeSomeOpenedDoor :: SomeDoor -> Maybe SomeDoor
closeSomeOpenedDoor (MkSomeDoor s d) = case s of
SOpened -> Just . fromDoor_ $ closeDoor d
SClosed -> Nothing
SLocked -> Nothing
lockAnySomeDoor :: SomeDoor -> SomeDoor
lockAnySomeDoor (MkSomeDoor s d) = fromDoor_ $ lockAnyDoor s d
```

Using an existential wrapper with a singleton makes this pretty simple – just a simple unwrapping and re-wrapping! Imagine having to re-implement all of these functions for a completely different type, and having to re-implement all of our previous `Door`

functions.

It’s important to remember that the secret ingredient here is the `Sing s`

we store inside `MkSomeDoor`

– it gives our pattern matchers the ability to deduce the `s`

type. Without it, the `s`

would be lost forever.

If `MkSomeDoor`

did not have the `Sing`

:

It would then be impossible to write `closeSomeOpenedDoor`

in a way that only works on opened doors:

```
closeSomeOpenedDoor :: SomeDoor -> Maybe SomeDoor
closeSomeOpenedDoor (MkSomeDoor d) =
-- is the door opened, closed, or locked?
-- there's no way to know!
-- curses, type erasure!
```

### The Link

It’s important to remember that our original separate-implementation `SomeDoor`

is, functionally, identical to the new code-reusing `Door`

. All of the contents are isomorphic with each other, and you could write a function converting one to the other. This is because *having an existentially quantified singleton is the same as having a value of the corresponding type.* Having an existentially quantified `SingDS s`

is *the same as* having a value of type `DoorState`

.

In fact, the *singletons* library gives us a direct existential wrapper:

```
-- from singletons (not the actual definition, just psuedo-code to demonstrate
-- what the constructors look like)
data SomeSing DoorState :: Type where
SomeSing :: Sing s -> SomeSing DoorState
```

There are three values of type `SomeSing DoorState`

:

```
SomeSing SOpened :: SomeSing DoorState
SomeSing SClosed :: SomeSing DoorState
SomeSing SLocked :: SomeSing DoorState
```

A value of type `SomeSing DoorState`

(which contains an existentially quantified `Sing s`

– a `SingDS`

) is *the same* as a value of type `DoorState`

. The two types are identical! (Or, well, isomorphic. As a fun exercise, write out the explicit isomorphism – the `SomeSing DoorState -> DoorState`

and the `DoorState -> SomeSing DoorState`

).

Our new `SomeDoor`

containing an existentially quantified `Sing s`

is the same as our first `SomeDoor`

containing just a `DoorState`

.

#### Why do we sing?

If they’re identical, why use a `Sing`

or the new `SomeDoor`

at all? Why not just use a `DoorState`

value?

One main reason (besides allowing code-reuse like we did earlier) is that *using the singleton lets us directly recover the type*. Essentially, a `Sing s`

not only contains whether it is Opened/Closed/Locked (like a `DoorState`

would), but also it contains it in a way that GHC can use to *bring it all back* to the type level.

The constructor `forall s. MkSomeDoor (Sing s) (Door s)`

essentially contains `s`

*with* `Door s`

. When you see this, you *should read this as* `forall s. MkSomeDoor s (Door s)`

(and, indeed, this is similar to how it is written in dependently typed languages.)

It’s kind of like how, when you’re used to reading Applicative style, you start seeing `f <$> x <*> y`

and reading it like `f x y`

. When you see `forall s. MkSomeDoor (Sing s) (Door s)`

, you should read (the pseudo-haskell) `forall s. MkSomeDoor s (Door s)`

. The role of `Sing s`

there is, like in Part 1, simply to be a run-time stand-in for the type `s`

itself.

So, for our original `Door s`

functions, we need to know `s`

at runtime – storing the `Sing s`

gives GHC exactly that. Once you get the `Sing s`

back, you can now use it in all of our type-safe functions from Part 1, and you’re back in type-safe land.^{2}

### Some Lingo

In the language of dependently typed programming, we call `SomeDoor`

a **dependent sum**, because you can imagine it basically as a sum type:

A three-way sum between a `Door 'Opened`

, a `Door 'Closed`

, and a `Door 'Locked`

, essentially. If you have a `SomeDoor`

, it’s *either* an opened door, a closed door, or a locked door. Try looking at this new `SomeDoor`

until you realize that this type is the same type as the previous `SomeDoor`

!

You might also see `SomeDoor`

called a **dependent pair** – it’s a “tuple” where the *type* of the second item (our `Door s`

) is determined by the *value* of the first item (our `Sing s`

).

In Idris, we could write `SomeDoor`

as a type alias, using its native dependent pair syntactic sugar, as `(s ** Door s)`

. The *value* of the first item reveals to us (through a pattern match, in Haskell) the *type* of the second.

### Types at Runtime

With this new tool, we finally have enough to build a function to “make” a door with the status unknown until runtime:

```
mkSomeDoor :: DoorState -> String -> SomeDoor
mkSomeDoor = \case
Opened -> fromDoor_ . mkDoor SOpened
Closed -> fromDoor_ . mkDoor SClosed
Locked -> fromDoor_ . mkDoor SLocked
```

```
ghci> let mySomeDoor = mkSomeDoor Opened "Birch"
ghci> :t mySomeDoor
SomeDoor
ghci> putStrLn $ case mySomeDoor of
MkSomeDoor SOpened _ -> "mySomeDoor was opened!"
MkSomeDoor SClosed _ -> "mySomeDoor was closed!"
MkSomeDoor SLocked _ -> "mySomeDoor was locked!"
mySomeDoor was opened!
```

Using `mkSomeDoor`

, we can truly pass in a `DoorState`

that we generate at runtime (from IO, or a user prompt, or a configuration file, maybe), and create a `Door`

based on it.

Take *that*, type erasure! :D

### The Existential Type

An *existentially quantified* type is one that is hidden to the user/consumer, but directly chosen by the producer. The producer chooses the type, and the user has to handle any possible type that the producer gave.

This is in direct contrast to the *universally quantified* type (which most Haskellers are used to seeing), where the type is directly chosen by the *user*. The user chooses the type, and the producer has to handle any possible type that the user asks for.

For example, a function like:

Is universally quantified over `a`

: The *caller* of `read`

gets to pick which type is given. The burden is on the implementor of `read`

to be able to handle whatever `a`

the user picks.

But, for a value like:

The type variable `s`

is existentially quantified. The person who *made* `myDoor`

picked what `s`

was. And, if you *use* `myDoor`

, you have to be ready to handle *any* `s`

they could have chosen.

In Haskell, there’s another way to express an existentially quantified type: the CPS-style encoding. This way is useful because it doesn’t require creating an intermediate helper data type. To help us understand it, let’s compare a basic function in both styles. We saw earlier `mkSomeDoor`

, which takes a `DoorState`

and a `String`

and returns an existentially quantified `Door`

in the form of `SomeDoor`

:

```
mkSomeDoor
:: DoorState
-> String
-> SomeDoor
mkSomeDoor s m = case s of
Opened -> fromDoor_ (mkDoor SOpened m)
Closed -> fromDoor_ (mkDoor SClosed m)
Locked -> fromDoor_ (mkDoor SLocked m)
```

The caller of the function can then break open the `SomeDoor`

and must handle whatever `s`

they find inside.

We can write the same function using a *CPS-style* existential instead:

```
withDoor
:: DoorState
-> String
-> (forall s. Sing s -> Door s -> r)
-> r
withDoor s m f = case s of
Opened -> f SOpened (mkDoor SOpened m)
Closed -> f SClosed (mkDoor SClosed m)
Locked -> f SLocked (mkDoor SLocked m)
```

With a Rank-N Type, `withDoor`

takes a `DoorState`

and a `String`

and a *function to handle a Door s polymorphically*. The caller of

`withDoor`

must provide a handler that can handle *any*

`s`

, in a uniform and parametrically polymorphic way. The function then gives the result of the handler function called on the resulting `Sing s`

and `Door s`

.```
ghci> withDoor Opened "Birch" $ \s _ -> case s of
SOpened -> "Opened door!"
SClosed -> "Closed door!"
SLocked -> "Locked door!"
Opened door!
```

The key to making this work is that your handler function *has to be polymorphic* over all possible `s`

s. This way, it can handle any potential `s`

that the producer gives. Essentially, the producer is “returning” an `s`

– existentially quantified.

### Reification

The general pattern we are exploring here is called **reification** – we’re taking a dynamic run-time value, and lifting it to the type level as a type (here, the type variable `s`

). Reification is often considered as the opposite of reflection, and we can imagine the two as being the “gateway” between the type-safe and unsafe world. In the dynamic world of a `DoorState`

term-level value, you have no type safety. You live in the world of `SomeDoor`

, `closeSomeOpenedDoor`

, `lockAnySomeDoor`

, etc. But, you can *reify* your `DoorState`

value to a *type*, and enter the type-safe world of `Door s`

, `closeDoor`

, `lockDoor`

, and `lockAnyDoor`

.

The *singletons* library automatically generates functions to directly reify `DoorState`

values:

```
toSing :: DoorState -> SomeSing DoorState
withSomeSing :: DoorState -> (forall s. Sing s -> r) -> r
withSomeSing :: DoorState -> (forall s. SDoorState s -> r) -> r
-- ^ using the convenience type synonym
```

The first one reifies a `DoorState`

as an existentially quantified data type, and the second one reifies one in CPS-style, without the intermediate data type.

We can actually use these to write `mkSomeDoor`

and `withDoor`

in a nicer way, without directly pattern matching on our constructors:

```
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/singletons/Door2.hs#L78-L83
mkSomeDoor :: DoorState -> String -> SomeDoor
mkSomeDoor ds = case toSing ds of
SomeSing s -> fromDoor s . mkDoor s
withDoor :: DoorState -> String -> (forall s. Sing s -> Door s -> r) -> r
withDoor ds m f = withSomeSing ds $ \s -> f s (mkDoor s m)
```

## Zooming Out

Alright! We’ve spent two blog posts going over a lot of different things in the context of our humble `Door s`

type. Let’s zoom out and take a large-scale look at how *singletons* (the design pattern, and the library) helps us in general.

### Sing

The crux of everything is the `Sing :: k -> Type`

kind-indexed injective type family. If you see a value of type `Sing s`

, you should really just think “a runtime witness for `s`

”. If you see:

You should read it as (in pseudo-Haskell)

This is seen clearly if we look at the partially applied type signatures:

If you squint, this kinda looks like:

And indeed, when we get real dependent types in Haskell, we will really be directly passing types (that act as their own runtime values) instead of singletons.

It is important to remember that `Sing`

is poly-kinded, so we can have `Sing 'Opened`

, but also `Sing 'True`

, `Sing 5`

, and `Sing '['Just 3, 'Nothing, 'Just 0]`

as well. `Sing x`

is an “synonym” for `SDoorState x`

when `x`

is a `DoorState`

, but `Sing x`

is a “synonym” for `SBool x`

is a `Bool`

. This is the real benefit of using the *singletons* library instead of writing our own singletons – we get to work uniformly with singletons of all kinds.

#### SingI

`SingI`

is a bit of typeclass trickery that lets us implicitly pass `Sing`

s to functions:

If you see:

These are *identical in power* to

Either way, you’re passing in the ability to get a runtime witness on `s`

– just in one way, it is asked for as an explicit argument, and the second way, it is passed in using a typeclass.

We can *convert* from `SingI s ->`

style to `SingI s =>`

style using `sing`

:

```
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/singletons/Door2.hs#L54-L67
lockAnyDoor_ :: SingI s => Door s -> Door 'Locked
lockAnyDoor_ = lockAnyDoor sing
fromDoor_ :: SingI s => Door s -> SomeDoor
fromDoor_ = fromDoor sing
```

And we can convert from `SingI s =>`

style to `SingI s ->`

style using `withSingI`

:

```
lockAnyDoor :: Sing s -> Door s -> Door 'Locked
lockAnyDoor s d = withSingI s (lockAnyDoor_ d)
fromDoor :: Sing s -> Door s -> SomeDoor
fromDoor s d = withSingI s (fromDoor_ d)
```

Again, the same function – just two different styles of calling them.

Here’s a nice trick to make this a little more clean: *singletons-2.4* offers a nice pattern synonym `Sing`

to reflect this symmetry. The pattern `Sing :: SingI a => Sing a`

acts both as a constructor and a witness for `SingI`

and `Sing`

:

```
doorStatus_ :: SingI s => Door s -> DoorState
doorStatus_ = doorStatus Sing -- using Sing constructs the Sing s
lockAnyDoor_ :: SingI s => Door s -> Door 'Locked
lockAnyDoor_ = lockAnyDoor Sing -- using Sing constructs the Sing s
lockAnyDoor :: Sing s -> Door s -> Door 'Locked
lockAnyDoor Sing d = lockAnyDoor_ d -- matching on Sing introduces SingI s
fromDoor :: Sing s -> Door s -> SomeDoor
fromDoor Sing d = fromDoor_ d -- matching on Sing introduces SingI s
```

### Reflection and Reification

Reflection is the process of bringing a type-level thing to a value at the term level (“losing” the type information in the process) and reification is the process of bringing a value at the *term level* to the *type level*.

Reflection and reification can be thought of as the gateways between the untyped/unsafe world and the typed/safe world. Reflection takes you from the typed world to the untyped world (from `Sing s`

to `DoorState`

) and reification takes you from the untyped world to the typed world (from `DoorState`

to `Sing s`

).

One limitation in Haskell is that there is no actual link between the type `DoorState`

and its *values* with the *kind* `DoorState`

with its *types*. Sure, the constructors have the same names, but the language doesn’t actually link them together for us.

#### SingKind

The *singletons* library handles this by using a typeclass with associated types to implement a generalized reflection and reification process. It gives us the `SingKind`

“kindclass”:

```
class SingKind k where -- `k` is a kind!
-- | Associate a kind k with its reflected type
type Demote k = (r :: Type)
-- | Reflect a singleton to its term-level value
fromSing :: Sing (a :: k) -> Demote k
-- | Reify a term-level value to the type level, as an existentially
-- quantified singleton
toSing :: Demote k -> SomeSing k
```

Instances of `SingKind`

are (promoted) *kinds* like `Bool`

-the-kind, `DoorState`

-the-kind, etc., and `Demote`

is an associated type/type family that associates each instance with the *type* it is promoted from. (Note – writing these type signatures requires the `-XTypeInType`

extension, which lets us treat kinds as types)

For example, remember how `data DoorState = Opened | Closed | Locked`

created the *type* `DoorState`

(with value constructors `Opened`

, `Closed`

, and `Locked`

), and also the *kind* `DoorState`

(with *type* constructors `'Opened`

, `'Closed`

, and `'Locked`

). Our *kind* `DoorState`

would be the instance of `SingKind`

, and `Demote DoorState`

would be the *type* `DoorState`

.

The reason we need an explicit `Demote`

associated type is, again, that GHC doesn’t actually link the type and its promoted kind. `Demote`

lets us explicitly specify what type a `Kind`

should expect its term-level reflected values to be. (And, like most things in this post, `Demote`

will hopefully one day become obsolete, along with the rest of `SingKind`

)

#### Examples

To illustrate explicitly, here is the automatically generated instance of `SingKind`

for the `DoorState`

*kind*:

```
instance SingKind DoorState where -- the *kind* DoorState
type Demote DoorState = DoorState -- the *type* DoorState
fromSing
:: Sing (s :: DoorState) -- the *kind* DoorState
-> DoorState -- the *type* DoorState
fromSing = \case
SOpened -> Opened
SClosed -> Closed
SLocked -> Locked
toSing
:: DoorState -- the *type* DoorState
-> SomeSing DoorState -- the *kind* DoorState
toSing = \case
Opened -> SomeSing SOpened
Closed -> SomeSing SClosed
Locked -> SomeSing SLocked
```

If you are unfamiliar with how associated types work, `type Demote DoorState = DoorState`

means that wherever we see `Demote DoorState`

(with `DoorState`

the *kind*), we replace it with `DoorState`

(the *type*). That’s why the type of our reflection function `fromSing :: Sing s -> Demote DoorState`

can be simplified to `fromSing :: Sing s -> DoorState`

.

Let’s take a look at the instance for `Bool`

, to compare:

```
-- Bool singletons have two constructors:
SFalse :: Sing 'False
STrue :: Sing 'True
instance SingKind Bool where -- the *kind* Bool
type Demote Bool = Bool -- the *type* Bool
fromSing
:: Sing (b :: Bool) -- the *kind* Bool
-> Bool -- the *type* Bool
fromSing = \case
SFalse -> False
STrue -> True
toSing
:: Bool -- the *type* Bool
-> SomeSing Bool -- the *kind* Bool
toSing = \case
False -> SomeSing SFalse
True -> SomeSing STrue
```

And a more sophisticated example, let’s look at the instance for `Maybe`

:

```
-- Maybe singletons have two constructors:
data SMaybe :: Maybe k -> Type where
SNothing :: SMaybe 'Nothing
SJust :: Sing x -> SMaybe ('Just x)
-- The syntax for declaring an instance for the kind-indexed type family
type instance Sing = SMaybe
instance SingKind k => SingKind (Maybe k) where -- the *kind* Maybe
type Demote (Maybe k) = Maybe (Demote k) -- the *type* Maybe
fromSing
:: Sing (m :: Maybe k) -- the *kind* Maybe
-> Maybe (Demote k) -- the *type* Maybe
fromSing = \case
SNothing -> Nothing
SJust sx -> Just (fromSing sx)
toSing
:: Maybe (Demote k) -- the *type* Maybe
-> SomeSing (Maybe k) -- the *kind* Maybe
toSing = \case
Nothing -> SomeSing SNothing
Just x -> case toSing x of
SomeSing sx -> SomeSing (SJust sx)
```

This definition, I think, is a real testament to the usefulness of having all of our singletons be unified under the same system. Because of how `SingKind`

works, `Demote (Maybe DoorState)`

is evaluated to `Maybe (Demote DoorState)`

, which is simplified to `Maybe DoorState`

. This means that if we have a way to reify `DoorState`

values, we also have a way to reify `Maybe DoorState`

values! And, if we have a way to reflect `DoorState`

singletons, we also have a way to reflect `Maybe DoorState`

singletons!

#### SomeSing

Throughout all of this, we utilize `SomeSing`

as a generic poly-kinded existential wrapper:

Basically, this says that `SomeSing k`

contains a `Sing x`

, where `x`

is of kind `k`

. This is why we had, earlier:

```
SomeSing :: Sing (s :: DoorState) -> SomeSing DoorState
SomeSing :: Sing (s :: Bool) -> SomeSing Bool
SomeSing :: Sing (s :: Maybe k) -> SomeSing (Maybe k)
```

If we use `SomeSing`

with, say, `SClosed`

, we get `SomeSing :: Sing 'Closed -> SomeSing DoorState`

. `SomeSing`

is an indexed type that tells us the *kind* of the type variable we existentially quantifying over. The value `SomeSing STrue`

would have the type `SomeSing Bool`

. The value `SomeSing (SJust SClosed)`

would have the type `SomeSing (Maybe DoorState)`

.

And, like for `SomeDoor`

, it is important to remember that `SomeSing a`

, for kind `a`

, is *isomorphic* to the type `a`

. This isomorphism is witnessed by `fromSing`

and `toSing`

, but here’s, visually, how things match up for `DoorState`

:

And how they match up for `Maybe Bool`

:

```
Nothing <~> SomeSing SNothing
Just False <~> SomeSing (SJust SFalse)
Just True <~> SomeSing (SJust STrue)
```

## Looking Forward

Between these first two parts, we explored a specific use case that would benefit from dependent types (simple phantom types for state transitions) and explored how the *singletons* and design pattern help us implement the functionality necessary to make things useful, and snuck in some concepts from dependently typed programming as well. We then took a step back to explore the *singletons* library in a more “universal” way, and saw how it is generalized to many different types.

The code is available here for you to play around with yourself!

Now that the basics are out of the way, in Part 3 we’ll jump deep into type-level programming and being able to lift our term-level functions on values up to become type-level functions, and how to use this to express complex relationships and enhance our code!

Let me know in the comments if you have any questions! I’m also usually idling on the freenode `#haskell`

channel, as well, as *jle`*.

And, again, I definitely recommend checking out the original singletons paper for a really nice technical overview of all of these techniques from the source itself.

### Exercises

Check out the sample code for solutions!

Let’s revisit our original redundant

`SomeDoor`

, compared to our final`SomeDoor`

:`-- source: https://github.com/mstksg/inCode/tree/master/code-samples/singletons/Door2.hs#L60-L91 data OldSomeDoor :: Type where OldMkSomeDoor :: DoorState -> String -> OldSomeDoor data SomeDoor :: Type where MkSomeDoor :: Sing s -> Door s -> SomeDoor`

To help convince yourself that the two are equal, write functions converting between the two:

`-- source: https://github.com/mstksg/inCode/tree/master/code-samples/singletons/Door2.hs#L93-L96 toOld :: SomeDoor -> OldSomeDoor fromOld :: OldSomeDoor -> SomeDoor`

**Avoid directly pattern matching on the singletons or constructors**. Instead, use*singletons*library tools like`toSing`

,`withSomeSing`

,`fromSing`

, etc.Previously, we had an

`unlockDoor`

function that took an`Int`

(the “password”) with a`Door 'Locked`

and returned a`Maybe (Door 'Closed)`

. It returns a`Door 'Closed`

(unlocked door) in`Just`

if an odd number was given, and`Nothing`

otherwise (a failed unlock)Use this to implement a that would return a

`SomeDoor`

. Re-use the “password” logic from the original`unlockDoor`

. If the door is successfully unlocked (with a`Just`

), return the unlocked door in a`SomeDoor`

. Otherwise,*return the original locked door*(in a`SomeDoor`

).`-- source: https://github.com/mstksg/inCode/tree/master/code-samples/singletons/Door2.hs#L99-L104 unlockDoor :: Int -> Door 'Locked -> Maybe (Door 'Closed) unlockDoor n (UnsafeMkDoor m) | n `mod` 2 == 1 = Just (UnsafeMkDoor m) | otherwise = Nothing unlockSomeDoor :: Int -> Door 'Locked -> SomeDoor unlockSomeDoor = ???`

Implement

`openAnyDoor'`

in the same style, with respect to`openAnyDoor`

:`-- source: https://github.com/mstksg/inCode/tree/master/code-samples/singletons/Door2.hs#L109-L118 openAnyDoor :: SingI s => Int -> Door s -> Maybe (Door 'Opened) openAnyDoor n = openAnyDoor_ sing where openAnyDoor_ :: Sing s -> Door s -> Maybe (Door 'Opened) openAnyDoor_ = \case SOpened -> Just SClosed -> Just . openDoor SLocked -> fmap openDoor . unlockDoor n openAnySomeDoor :: Int -> SomeDoor -> SomeDoor openAnySomeDoor = ???`

Remember to re-use

`openAnyDoor`

.Write the

`SingKind`

instance for the promoted kind of a custom list type:`-- source: https://github.com/mstksg/inCode/tree/master/code-samples/singletons/Door2.hs#L124-L132 data List a = Nil | Cons a (List a) data SList :: List a -> Type where SNil :: SList 'Nil SCons :: Sing x -> SList xs -> SList ('Cons x xs) type instance Sing = SList instance SingKind k => SingKind (List k) where type Demote (List k) = ??? fromSing :: Sing (xs :: List k) -> List (Demote k) fromSing = ??? toSing :: List (Demote k) -> SomeSing (List k) toSing = ???`

Note that the built-in singletons for the list type also uses these same constructor names, for

`[]`

and`:`

.

And also a not-so-obvious fourth type,

`forall s. Door s`

, which is a subtype of all of those three!↩︎You might have noticed I was a bit sneaky by jumping straight

`SomeDoor`

when we already had a perfectly good “I don’t care” option. We used it last post!This does work!

`lockAnyDoor`

takes a`Door s`

and doesn’t “care” about what`s`

it gets (it’s parametrically polymorphic).So, this normal “parametrically polymorphic” way is how we have, in the past, treated functions that

*can take*a`Door`

with an`s`

we don’t want the type system to care about. However, the reason we need`SomeDoor`

and existentially quantified types is for the situation where we want to*return*something that we want to the type system to not care about.↩︎