[ ghc-Bugs-1075259 ] Wrong overlapped/missing pattern warnings

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Bugs item #1075259, was opened at 2004-11-29 14:00
Message generated for change (Comment added) made by simonpj
You can respond by visiting: 
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Category: Compiler (Parser)
Group: 6.2.2
Status: Open
Resolution: None
Priority: 5
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 (maeder at tzi.de)


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>Comment By: Simon Peyton Jones (simonpj)
Date: 2004-12-13 09:49

Message:
Logged In: YES 
user_id=50165

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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