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