Re: The Proper Definition of (evaluate :: a -> IO a)

On Mon, 07 May 2007, Malcolm Wallace wrote:

Isaac Dupree wrote:

...

The obvious "evaluate x = x `seq` return x" fails one of the following laws for evaluate:

evaluate x `seq` y ==> y

I'm not sure why anyone thinks this "law" should hold, since it completely changes the known semantics of `seq`. A more accurate law would be:

evaluate x `seq` y ==> if x==_|_ then _|_ else y

You seem to be assuming that evaluate ⊥ = ⊥, which is not necessarily the case.

Where did you find the erroneous version?

Presumably he found it by reading the documentation for evaluate: http://haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception.htm... -- /NAD