>Consider that calling > >> id undefined > >requires evaluating undefined before you can call id. The program will >"crash" before you ever call id. Of >course, the identity function "should" >have produced a value that crashed in exactly the same way. But we >never got >there.
In which sense does id need to evaluate its argument? If evaluating the statement "id undefined" in ghci, then the print function (and ultimately the show function) evaluates the undefined. I suppose that if the show function does not evaluate its argument then there will be no error, right? /Jonas On 24 May 2010 09:05, Alexander Solla <a...@2piix.com> wrote: > > On May 23, 2010, at 2:53 AM, Lennart Augustsson wrote: > >> BTW, the id function works fine on bottom, both from a semantic and >> implementation point of view. > > Yes, but only because it doesn't work at all. Consider that calling > >> id undefined > > requires evaluating undefined before you can call id. The program will > "crash" before you ever call id. Of course, the identity function "should" > have produced a value that crashed in exactly the same way. But we never > got there. > > It works in same way (==) works on bottoms. Vacuously. Indeed, functions > only work on values. There is no value of type (forall a . a). Ergo, id > does not work on undefined, as a function applying to a value. It is happy > coincidence that the behavior of the runtime can be made mostly compatible > with our familiar semantics. But not entirely. > > Consider: > > *Main> let super_undefined = super_undefined > *Main> undefined == super_undefined > *** Exception: Prelude.undefined > *Main> super_undefined == super_undefined > (wait forever) > *Main> id super_undefined > (wait forever) > *Main> id undefined > *** Exception: Prelude.undefined > > From a mathematical perspective, super_undefined is Prelude.undefined. But > it takes some runtime level hackery to make undefined an exceptional > "value". Without it, super_undefined loops, despite the fact that their > definitions are exactly the same (The "value" defined by itself). > > I did not suggest those are the wrong semantics. Only that the study of > their semantics doesn't belong to the study of Haskell as a logic/language. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
