[Haskell-cafe] Why this doesn't type checked

[Daniel Fischer]

On Sunday 27 June 2010 21:52:18, Victor Nazarov wrote:
> I've allways found code like
>
> > -- maxBound (undefined :: Int)
>
> a bit strange

Well, that would indeed be a bit strange since

maxBound :: (Bounded a) => a

and function types aren't instances of Bounded, so it'd be

maxBound :: Int
maxBound :: Char
maxBound :: Bool
...

> as any usage of undefined is.
> Being Ruby on Rails developer
> I've personally found that one of the main
> Rails motos is being as readable as possible.

That's good.

> Code must be as close to english as possible.

That not, not always, anyway. Mathematical algorithms for example tend to 
be obfuscated by englishifying.

> Embeded DSLs like rspec are made mostly to
> made specs as close to english as possible.
>


> What we get with this instances is following code.
>
> > main =
> >   do print (sizeof :: Sizeof Word16)
>
> Let's try it.
>
> $ runhaskell this.lhs
> this.lhs:78:14:
>     Couldn't match expected type `Int'
>            against inferred type `Sizeof sizeable'
>       NB: `Sizeof' is a type function, and may not be injective
>     In the first argument of `print', namely
>         `(sizeof :: Sizeof Word16)'
>     In the expression: print (sizeof :: Sizeof Word16)
>     In the expression: do { print (sizeof :: Sizeof Word16) }

Right. Since Sizeof Word8 is Int too, the type can't help determining the 
value.

>
> What can I do with this code to make it type-check?

newtype Size a = Size { unSize :: Int }

class Sizeable a where
    sizeof :: Size a

instance Sizeable Word8 where
    sizeof = Size 1

instance Sizeable Word16 where
    sizeof = Size 2

...

main = print . unSize $ sizeof :: Size Word16