[Haskell-beginners] Re: Forcing evalation in the IO Monad

[Daniel Fischer]

Am Freitag 16 April 2010 17:09:07 schrieb Philip Scott:
> Philip Scott wrote
>
> > The documentation says that, but it does appear to go deeper than just
> > one level:
> >
> > Debug.Trace Prelude Control.Exception> let a = trace "Hello" 42
> > Debug.Trace Prelude Control.Exception> let b = trace "Frank" (a * 2)
> > Debug.Trace Prelude Control.Exception> evaluate b
> > Frank
> > Hello
> > 84
> >
> > Perhaps it specifies WHNF in case you 'evaluate' something which
> > doesn't have a HNF (like a partially applied function?).
>
> It is actually quite interesting what it does in this case:
>  > let f x = trace "jingle" (x * 2)
>  > a <- evaluate (f)
>  >
>  > :t f
>
> f :: (Num a) => a -> a
>
>  > :t a
>
> a :: Integer -> Integer
>
> So it does some sort of evaluation - it has decided we're going to be
> using Integers instead of Nums. I wonder what it is up to there.

Without impredicative polymorphism (whatever that is), you can't get a 
polymorphic value from an action, so while

Prelude Debug.Trace Control.Exception> :t evaluate f          
evaluate f :: (Num a) => IO (a -> a)

, when you bind the result of evaluate f to a name, that is assigned a 
monomorphic type. Without a type signature, the type variable a is 
defaulted to Integer. You can (until 6.14)
          
Prelude Debug.Trace Control.Exception> :set -XImpredicativeTypes
Prelude Debug.Trace Control.Exception> u <- evaluate f :: IO (forall a. Num 
a => a -> a)                                                                            

<no location info>:
    Warning: -XImpredicativeTypes is deprecated: impredicative polymorphism 
will be simplified or removed in GHC 6.14                                                         
Prelude Debug.Trace Control.Exception> :t u                                  
u :: (Num a) => a -> a                                                                 
Prelude Debug.Trace Control.Exception> u 4 :: Int                            
jingle                                                                                 
8                                                                                      
Prelude Debug.Trace Control.Exception> u 4 :: Double
jingle                                                        
8.0                                                           
Prelude Debug.Trace Control.Exception> u 4 :: Rational
jingle                                                          
8 % 1