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

[Nils Anders Danielsson]

On Mon, 07 May 2007, Malcolm Wallace <Malcolm.Wallace at cs.york.ac.uk> wrote:

> Isaac Dupree <isaacdupree at charter.net> 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.html#7

-- 
/NAD