Custom Query

Reported by nobody, 11 years ago.

When some of the patterns in a function definition or
case expression contain guards, the compiler gives
false warnings about non-exhaustive pattern matches.  A
pity, because this kind of warning is very useful
indeed!

For example, I get 
 >ghc --make -Wall See.hs
 ghc-5.02.1: chasing modules from: See.hs
 Compiling See              ( See.hs, ./See.o )

 See.hs:17: Warning: Pattern match(es) are
non-exhaustive
     In the definition of `insert':
 	Patterns not matched: _ _ (TREE _ _ _ _)

 See.hs:41: Warning: Pattern match(es) are
non-exhaustive
    In a case alternative EMPTY: Patterns not matched:
TREE _ _ _ _

for the attached file.

[I am not sure filling in the e-mail address box had
any effect on the last two complaints I submitted, so I
repeat my address here:
feliks@crt.se]

Reported by nobody, 11 years ago.

An atttempt to attach a file to the last submission
resulted in the
following message from Netscape (?):
  Fatal error: Call to a member function on a
non-object in   common/tracker/ArtifactFile.class on
line 106

So I include the file below (the line numbers will be
changed),

================================
module See  where

data  Tree key value  
             =  EMPTY
             |  TREE key value (Tree key value) (Tree
key value)


insert ::  ( Ord key ) =>
             key -> value ->            --  insert this
new key and value
             Tree key value ->          --  into this
tree
             Maybe (Tree key value)  --  to form (Just
new tree ),
                                                 --    
or Nothing if key is present

insert k v EMPTY  =  Just (TREE k v EMPTY EMPTY)

insert k v (TREE k' v' left right)

    |  k < k'   =  case  insert k v left  of
                     Nothing    -> Nothing
                     Just left' -> Just (TREE k' v'
left' right)

    |  k > k'   =  case  insert k v right  of
                     Nothing     -> Nothing
                     Just right' -> Just (TREE k' v'
left right')

    |  k == k'  =  Nothing


-- Same with a case expression:

insert' ::  ( Ord key ) =>
           key -> value -> Tree key value -> Maybe
(Tree key value)

insert' k v t  =

   case  t  of

       EMPTY  -> Just (TREE k v EMPTY EMPTY)

       (TREE k' v' left right)

           |  k < k'   ->  case  insert' k v left  of
                             Nothing    -> Nothing
                             Just left'   -> Just (TREE
k' v' left' right)

           |  k > k'   ->  case  insert' k v right  of
                             Nothing     -> Nothing
                             Just right'  -> Just (TREE
k' v' left right')

           |  k == k'   ->  Nothing


-- Nonsensical, but patterns without guards:

insert'' :: 
    key -> value -> Tree key value -> Maybe (Tree key
value)

insert'' _ _ EMPTY                  =  Nothing

insert'' k v (TREE _ _ left right)  =  Just (TREE k v
left right)

======================================

Reported by nobody, 11 years ago.

On p. 53 the manual says (in the description of
"-Wall"):

 To turn off any warning, simply give the corresponding
-fno-warn-... option on the command line.

One of the options is  -fwarn-missing-signatures (p.
54).  Yet the compiler produces an error message:

 > ghc --make -odir ../ghc -hidir ../ghc
testDeclarationsMain.hs -o testDeclarations -Wall
-fnowarn-missing-signatures
  ghc-5.02.1: unrecognised flag:
-fnowarn-missing-signatures