wiki:TypeNats/BasicExamples

Version 1 (modified by diatchki, 3 years ago) (diff)

--

Here is how we can use this API 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 :: TypeNat n => Integer -> Maybe (Nat n)
integerToMaybeNat x = check nat
  where check y = if x == natToInteger y then Just y else Nothing

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, and providing explicit type signatures.

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

instance TypeNat n => Read (Nat n) where
  readsPrec p x       = do (x,xs) <- readsPrec p x
                           case integerToMaybeNat x of
                             Just n  -> [(n,xs)]
                             Nothing -> []