Bugs item #1075259, was opened at 2004-11-29 14:00
Message generated for change (Comment added) made by ginge
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Category: Compiler (Parser)
Group: 6.2.2
Status: Open
Resolution: None
Priority: 2
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: Wrong overlapped/missing pattern warnings
Initial Comment:
compiling:
module Overlap where
f (n+1) = 2
f 0 = 1
emits wrongly:
Warning: Pattern match(es) are overlapped
In the definition of `f': f 0 = ...
The Patterns are disjoint, aren't they? At least "f 0"
yields "1" when evaluated and negative inputs for f are
rejected. However the warning is not emitted if the two
equations are given in reversed order.
Christian ([EMAIL PROTECTED])
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Comment By: Neil Mitchell (ginge)
Date: 2005-12-03 16:15
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When adding -fwarn-simple-patterns to the command line, all
3 of the previous examples give an additional 2 warning's,
i.e. ex1 and ex2 give two identical warnings, and ex3 gives
3 identical warnings.
Warning: Pattern match(es) are non-exhaustive
In a case alternative: Patterns not matched: []
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Comment By: Simon Peyton Jones (simonpj)
Date: 2005-12-02 09:12
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Another example from Neil Mitchell
I have been playing around with -fwarn-incomplete-patterns
under GHC
6.4.1 on Windows.
-- no warning
ex1 x = ss
where (s:ss) = x
-- no warning
ex2 x = let (s:ss) = x in ss
-- Warning: Pattern match(es) are non-exhaustive
-- In a case alternative: Patterns not matched:
[]
ex3 x = case x of ~(s:ss) -> ss
I have translated all 3 functions using the rules supplied
in the Haskell 98 report, so they all have the same
meaning, but only one gives an error. Is it intentional to
ignore where/let pattern matches?
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Comment By: Simon Peyton Jones (simonpj)
Date: 2004-12-13 09:49
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Here's another example, from Peter White
When I compile the following module with the -Wall option on
ghc v6.2.1
I get warnings:
Warning: Pattern match(es) are non-exhaustive
In a record-update construct: Patterns not matched D2.
The warnings occur at both of the indicated places in the
module.
Since the functions both handle all the cases of the data
type D, it
seems the warning should not be given.
data D = D1 { f1 :: Int } | D2
-- Use pattern matching in the argument
f :: D -> D
f d1@(D1 {f1 = n}) = d1 { f1 = f1 d1 + 1 } -- Warning here
f d = d
-- Use case pattern matching
g :: D -> D
g d1 = case d1 of
D1 { f1 = n } -> d1 { f1 = n + 1 } -- Warning here
also
D2 -> d1
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