#5894: Add generalization of Data.Map.unionWith with (a -> b -> c) as the
combining function
------------------------------+---------------------------------------------
Reporter: joeyadams | Owner:
Type: feature request | Status: new
Priority: normal | Component: libraries (other)
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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The signature of
[http://hackage.haskell.org/packages/archive/containers/latest/doc/html
/Data-Map.html#v:unionWith Data.Map.unionWith] is:
{{{
unionWith :: Ord k => (a -> a -> a) -> Map k a -> Map k a -> Map k a
}}}
[http://hackage.haskell.org/packages/archive/containers/latest/doc/html
/Data-Map.html#v:intersectionWith intersectionWith], on the other hand,
has a more general signature:
{{{
intersectionWith :: Ord k => (a -> b -> c) -> Map k a -> Map k b -> Map k
c
}}}
The reason the combining function in {{{unionWith}}} is constrained to
{{{(a -> a -> a)}}} is because of the possibility of values in one map not
present in the other. {{{unionWith}}} only applies the combining function
for values present in both maps; it uses existing values for the rest.
I'd like to be able to write this function:
{{{
-- | Combine two maps. Substitute 'Nothing' for any value present in only
one
-- of the maps.
unionMaybes :: Ord k => Map k a -> Map k b -> Map k (Maybe a, Maybe b)
}}}
However, it's awkward with {{{unionWith}}}:
{{{
unionMaybes left right =
Map.unionWith combine (Map.map mkLeft left) (Map.map mkRight
right)
where
mkLeft l = (Just l, Nothing)
mkRight r = (Nothing, Just r)
combine (Just l, Nothing) (Nothing, Just r) = (Just l, Just r)
combine _ _ = error "Shouldn't happen"
}}}
It'd be nicer to have a generalization of {{{unionWith}}} that can
transform values only present in one of the maps. Here is such a
function, implemented in terms of {{{unionWith}}}:
{{{
-- | Like 'Map.unionWith', but apply a function even when a key is present
in
-- only one of the maps.
--
-- @'Map.unionWith' = 'combine' 'id' 'id'@
combine :: Ord k
=> (a -> c) -- ^ For keys only present in the left map
-> (b -> c) -- ^ For keys only present in the right map
-> (a -> b -> c) -- ^ For keys present in both maps
-> Map k a -- ^ Left map
-> Map k b -- ^ Right map
-> Map k c
combine onLeft onRight onBoth left right =
Map.map apply $
Map.unionWith combine_ (Map.map LeftOnly left) (Map.map RightOnly
right)
where
combine_ (LeftOnly l) (RightOnly r) = LeftRight l r
combine_ _ _ =
error "combine: internal error (Data.Map.unionWith is not
combining values correctly)"
apply (LeftOnly l) = onLeft l
apply (RightOnly r) = onRight r
apply (LeftRight l r) = onBoth l r
data Combine l r = LeftOnly l
| RightOnly r
| LeftRight l r
}}}
This way, {{{unionMaybes}}} can be written as:
{{{
unionMaybes :: Ord k => Map k a -> Map k b -> Map k (Maybe a, Maybe b)
unionMaybes = combine (\l -> (Just l, Nothing))
(\r -> (Nothing, Just r))
(\l r -> (Just l, Just r))
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5894>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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