[Haskell-cafe] Re: Debugging partial functions by the rules
jmaessen at alum.mit.edu
Wed Nov 15 17:11:36 EST 2006
On Nov 15, 2006, at 3:21 AM, oleg at pobox.com wrote:
> Donald Bruce Stewart wrote:
>> So all this talk of locating head  and fromJust failures got me
>> 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.
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
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