On Nov 15, 2006, at 3:21 AM, [EMAIL PROTECTED] wrote:
Donald Bruce Stewart wrote:So all this talk of locating head [] and fromJust failures got me thinking: Couldn't we just use rewrite rules to rewrite *transparently* all uses of fromJust to safeFromJust, tagging the call site with a location?I'm sorry for shifting the topic: I'm wondering if, rather than trying to make an error message more informative, we ought to make sure that no error will ever arise? ... This topic has been discussed at length on this list. It seems that the discussion came to the conclusion that eliminating head of the empty list error is possible and quite easy in Haskell.http://www.haskell.org/pipermail/haskell-cafe/2006-September/ 017915.html
But this code contains a function with_non_empty_list (or perhaps with_nonempty_list or withNonemptyList or...) which has the same confusing failure mode as the examples under discussion. Fundamentally, if we try to package up "check for failure" in a function, whether the function does something useful as well (head, tail, fromJust) or not (withNonemptyList), we miss out on useful contextual information when our program fails.
In addition, we have this rather nice assembly of functions which work on ordinary lists. Sadly, rewriting them all to also work on NonEmptyList or MySpecialInvariantList is a nontrivial task.
Which isn't to say that I disapprove of this style: check your invariants early, maintain them as you go. I'm quite enjoying the escH paper, but I get through about a column per day between compiles. :-)
-Jan-Willem Maessen
smime.p7s
Description: S/MIME cryptographic signature
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
