At 18:38 14/10/03 -0700, Brandon Michael Moore wrote:
On Mon, 13 Oct 2003, Graham Klyne wrote:
Results in a fairly obvious type error: I'd need to have a way to say that vbMap is applied to the value under construction. Experience with Java would suggest maybe something like this: [[ , vbEnum = map (\v -> (v,fromJust (vbMap this v))) $ ]] but of course Haskell isn't Java.
The natural way to do this is to apply vbMap to the value under construction, Haskell being lazy and all. Of course this requires naming the variable under construction it hardly makes a difference when there is only one subexpression.
[[ joinVarBindings :: (Eq a) => VarBinding a b -> VarBinding a b -> VarBinding a b joinVarBindings vb1 vb2 | vbNull vb1 = vb2 | vbNull vb2 = vb1 | otherwise = let vb = VarBinding { vbMap = headOrNothing . filter isJust . flist [ vbMap vb1, vbMap vb2 ] , vbEnum = map (\v -> (v,fromJust (vbMap vb))) $ boundVars vb1 `union` boundVars vb2 , vbNull = False } ]]
This should work.
Almost! Using 'this' to label the value being defined, I had to add an "in this" at the end, thus: [[ joinVarBindings :: (Eq a) => VarBinding a b -> VarBinding a b -> VarBinding a b joinVarBindings vb1 vb2 | vbNull vb1 = vb2 | vbNull vb2 = vb1 | otherwise = let this = VarBinding { vbMap = headOrNothing . filter isJust . flist [ vbMap vb1, vbMap vb2 ] , vbEnum = map (\v -> (v,fromJust (vbMap this v))) $ boundVars vb1 `union` boundVars vb2 , vbNull = False } in this ]] (Getting the layout just right here proved to be a minor challenge) I hadn't thought of using 'let' in this way, and this led me to wondering if a form of do might be used... Out of curiosity, I also tried this: [[ joinVarBindings :: (Eq a) => VarBinding a b -> VarBinding a b -> VarBinding a b joinVarBindings vb1 vb2 | vbNull vb1 = vb2 | vbNull vb2 = vb1 | otherwise = do { let this = VarBinding { vbMap = headOrNothing . filter isJust . flist [ vbMap vb1, vbMap vb2 ] , vbEnum = map (\v -> (v,fromJust (vbMap this v))) $ boundVars vb1 `union` boundVars vb2 , vbNull = False } ; this } ]] which according to the Haskell report (p29) should translate to just the previous example, but Hugs reports a "cannot justify constraints" error ... it seems to want VarBindings to be a Monad, even though the translation to a non-do form can be achieved without invoking any monadic operator. Is this a bug? Thanks! #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact