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

[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.