Hello.
I'm having some trouble trying to understand exactly what's behind the rule for pattern-matching with data constructors. The code I'm having trouble with is similar to this:
f (C p1 p2 (C2 p3 p4)) = ...
f _ = False
What happens is if f is called with (C p1 p2 (NOT_C2 ...)), I get a program error, and not False.
I dug up the semantics of pattern-matching on the Report, which says, concerning data constructors:
"4. Matching a non- bot value can occur against three kinds of refutable patterns:
(a) Matching a non- bot value against a pattern whose outermost component is a constructor defined by data fails if the value being matched was created by a different constructor. If the constructors are the same, the result of the match is the result of matching the sub-patterns left-to-right against the components of the data value: if all matches succeed, the overall match succeeds; the first to fail or diverge causes the overall match to fail or diverge, respectively."
The "overall" match refers to the whole function or the equation being considered? More specifically, in my example: if matching against C succeeds, and matching against C2 fails, should the implementation try the next equation or give a program error?
I used to think that if C succeeded and C2 failed, the next equation would be tried, but it seems that is not what is happening (at least with Hugs).
One workaround seems to be to change (C2 p3 p4) into a var pattern, and match the C2 constructor inside a separate case statement, but this would clutter up the code considerably (maybe not in the example case, but I intended to pattern-match against some reasonably complex structures)
Is there any solution?
Thank you in advance, and thanks for helping,
Carlos Eduardo Scheidegger
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