Version 20 (modified by diatchki, 7 years ago) (diff)

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## Type-Level Naturals

There is a new kind, `Nat`. It is completely separate from GHC's hierarchy of sub-kinds, so `Nat` is only a sub-kind of itself.

The inhabitants of `Nat` are an infinite family of (empty) types, corresponding to the natural numbers:

```0, 1, 2, ... :: Nat
```

These types are linked to the value world by a small library with the following API:

```module GHC.TypeNats where
```

## Singleton Types

We relate type-level natural numbers to run-time values via a family of singleton types:

```data Nat (n :: Nat)

nat          :: NatI n => Nat n
natToInteger :: Nat n -> Integer
```

The only "interesting" value of type Nat n is the number n. Technically, there is also an undefined element. The value of a singleton type may be named using nat, which is a bit like a "smart" constructor for Nat n. Note that because nat is polymorphic, we may have to use a type signature to specify which singleton we mean. For example:

```> natToInteger (nat :: Nat 3)
3
```

One may think of the smart constructor nat as being a method of a special built-in class, NatI:

```class NatI n where
nat :: Nat n

instance NatI 0 where nat = "singleton 0 value"
instance NatI 1 where nat = "singleton 1 value"
instance NatI 2 where nat = "singleton 2 value"
etc.
```

The name NatI is a mnemonic for the different uses of the class:

• It is the introduction construct for 'Nat' values,
• It is an implicit parameter of kind 'Nat' (this is discussed in more detail bellow)

## Examples

Here is how we can use the basic primitives to define a `Show` instance for singleton types:

```instance Show (Nat n) where
showsPrec p n = showsPrec p (natToInteger n)
```

A more interesting example is to define a function which maps integers into singleton types:

```integerToMaybeNat :: NatI n => Integer -> Maybe (Nat n)
integerToMaybeNat x = check nat
where check y = if x == natToInteger y then Just y else Nothing
```

It checks that the value argument `x`, passed at runtime, matches the statically-expected value, returning `Nothing` if not, and a typed singleton if so.

The implementation of `integerToMaybeNat` is a little subtle: by using the helper function `check`, we ensure that the two occurrences of `nat` (aka `y`) both have the same type, namely `Nat n`. There are other ways to achieve the same, for example, by using scoped type variables, thus:

```integerToMaybeNat :: forall n. NatI n => Integer -> Maybe (Nat n)
integerToMaybeNat x
| x == natToInteger (nat :: Nat n) = Just nat
| otherwise                        = Nothing
```

Now, we can use `integerToNat` to provide a `Read` instance for singleton types:

```instance NatI n => Read (Nat n) where
case integerToMaybeNat x of
Just n  -> [(n,xs)]
Nothing -> []
```

## Implicit vs. Explicit Parameters

There are two different styles of writing functions which need the integer corresponding to a type level natural.

One approach is to use an explicit parameter of type `Nat n`. For example:

```memset :: Storable a => ArrPtr n a -> a -> Nat n -> IO ()
memset = ...
```

This style is, basically, a more typed version of what is found in many standard C libraries. Callers of this function have to pass the size of the array explicitly, and the type system checks that the size matches that of the array. When defining `memset` we can just use `natToInteger` on the `Nat n` parameter to get the actual value of the array size.

Another approach is to let the system infer the parameter by using the class `NatI`. For example:

```memsetAuto :: (Storable a, NatI n) => ArrPtr n a -> a -> IO ()
```

In this style, the caller of the function does not need to provide the size of the array explicitly. Instead, it is computed automatically from the type of the array. When defining `memsetAuto` we can use `nat`, the method of `NatI`, to get access to the value corresponding to the type level natural.

When using the implicit style, it is important that the type of `nat` is specified precisely. Failing to do so typically results in ambiguity errors (i.e., GHC does not know which integer it should use). Another common mistake is to forget that 'nat' is a polymorphic value and so every time it is used it may refer to a different value.

An easy way to avoid such problems is to implement the implicit style functions in terms of the explicit ones. For example, we can implement `memsetAuto` like this:

```memsetAuto arr val = memset arr val nat
```