Ticket #3453: check

Sun Aug 23 09:46:11 BST 2009  jon.fairbairn@cl.cam.ac.uk
  * Add MonadPlus check function
  Generalisation of guard. Of use in writing functions such as this:
  
  readMaybe :: Read a => String -> Maybe a
  readMaybe
      = join . fmap no_trailing_garbage . listToMaybe . reads
        where no_trailing_garbage = fmap fst . check (all isSpace . snd)
  

New patches:

[Add MonadPlus check function
jon.fairbairn@cl.cam.ac.uk**20090823084611
 Ignore-this: 34c8e9768f9cbb9512f207d357fb587d
 Generalisation of guard. Of use in writing functions such as this:
 
 readMaybe :: Read a => String -> Maybe a
 readMaybe
     = join . fmap no_trailing_garbage . listToMaybe . reads
       where no_trailing_garbage = fmap fst . check (all isSpace . snd)
 
] {
hunk ./Control/Monad.hs 60
     -- ** Conditional execution of monadic expressions
 
     , guard         -- :: (MonadPlus m) => Bool -> m ()
+    , check         -- :: (MonadPlus m) => (a -> Bool) -> a -> m a
     , when          -- :: (Monad m) => Bool -> m () -> m ()
     , unless        -- :: (Monad m) => Bool -> m () -> m ()
 
hunk ./Control/Monad.hs 269
 unless            :: (Monad m) => Bool -> m () -> m ()
 unless p s        =  if p then return () else s
 
+{- | conditional injection into a MonadPlus
+
+generalisation of @guard@:
+takes a predicate and an item and returns the item if the
+predicate is true for that item. For example
+
+> check (not . isSpace) c :: Maybe Char
+
+will return @Just c@ if @c@ is not a space, or @Nothing@ otherwise
+
+-}
+check :: (MonadPlus m) => (a -> Bool) -> a -> m a
+check p a
+    | p a = return a
+    | otherwise = mzero
+-- we expect these to be true:
+-- guard == flip (check . const)
+-- List.filter = (concat .) . map . check
+-- which suggests that
+-- (join .) . liftM . check :: (MonadPlus m) => (a1 -> Bool) -> m a1 -> m a1
+-- is an interesting function, call it mfilter; now
+-- check p == mfilter p . return
+
 -- | Promote a function to a monad.
 liftM   :: (Monad m) => (a1 -> r) -> m a1 -> m r
 liftM f m1              = do { x1 <- m1; return (f x1) }
}

Context:

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] 
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] 
[TAG 2009-06-25
Ian Lynagh <igloo@earth.li>**20090625160056] 
Patch bundle hash:
0eddd5471a0765babc52a8c452ed7a3948742049