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haskellbounded-types

How to get the maxBound of a type that has a Bounded instance

The following Haskell code does not compile:

getMaxBound :: (Bounded a) => a -> a
getMaxBound _ = maxBound :: a

The error I get is the following:

Could not deduce (Bounded a1) arising from a use of ‘maxBound’
  from the context: Bounded a
    bound by the type signature for:
               getMaxBound :: Bounded a => a -> a
    at rot13.hs:3:1-36

Why can't I get the maxBound of a bounded type in Haskell?

Ideally, I would use this function like this:

getMaxBound 3
> 9223372036854775807

getMaxBound 'c'
> '\1114111'

I feel like every time I have any type a which has a Bounded instance, I should be able to get the maxBound of that type.

What am I missing?

Solution

  • Type signature is sufficient

    Since the signature already restricts the type, you can drop the :: a part in the function body:

    getMaxBound :: Bounded a => a -> a
getMaxBound _ = maxBound

    The type signature specifies that type of the input of the function is the same as the type of the output of the function.

    For example:

    Prelude> getMaxBound (3 :: Int)
9223372036854775807
Prelude> getMaxBound 'c'
'\1114111'
Prelude> getMaxBound True
True
Prelude> getMaxBound LT
GT

    Using the ScopedTypeVariables extension

    We can also use the ScopedTypeVariables extension [Haskell-wiki], and then implement it with a reference to the a type variable:

    {-# LANGUAGE ScopedTypeVariables #-}

getMaxBound :: forall a . Bounded a => a -> a
getMaxBound _ = maxBound :: a