Hi,
I'm currently working on a GHC extension called "monad comprehensions"
[1]. Typechecking for generators ("pat <- rhs") already works, but
filters are a bit more tricky. Basicly, it works like that:
A monad comprehension...
[ body | E ] -- E :: Bool
...should desugar to:
Control.Monad.guard E >> return body
In the typechecker I typecheck E for type Bool, and see if we're in a
MonadPlus by typechecking "guard". But I have to "pass" this
guard-then-op function to the desugarer, so I created a new expression
"then_op' = (>>) . guard" and pass this function to the ExprStmt
constructor.
The code in typecheck/TcMatches.lhs looks currently like this:
tcMcStmt _ (ExprStmt rhs then_op _) res_ty thing_inside = do
{ -- Typecheck rhs on type Bool
rhs' <- tcMonoExpr rhs boolTy
-- Deal with rebindable syntax:
-- then_op :: rhs_ty -> new_res_ty -> res_ty
-- See notes in tcDoStmt.
-- After this we redefine then_op to have the following type:
-- then_op' :: Bool -> new_res_ty -> res_ty
-- then_op' = (>>) . guard
; rhs_ty <- newFlexiTyVarTy liftedTypeKind
; new_res_ty <- newFlexiTyVarTy liftedTypeKind
; let then_ty = mkFunTys [rhs_ty, new_res_ty] res_ty
guard_ty = mkFunTys [boolTy] rhs_ty
comp_ty = mkFunTys [then_ty, guard_ty, boolTy, new_res_ty]
res_ty
; then_op <- tcSyntaxOp MCompOrigin then_op then_ty
; guard_op <- tcSyntaxOp MCompOrigin (HsVar guardMName)
guard_ty
; compose_op <- tcSyntaxOp MCompOrigin (HsVar composeName)
comp_ty
; let then_op' = HsApp (nlHsApp (noLoc compose_op)
(noLoc then_op))
(noLoc guard_op)
; thing <- thing_inside new_res_ty
; return (ExprStmt rhs' then_op' boolTy, thing) }
Is this a valid approach? Should I move the "(>>) . guard" function
somewhere else? I had a look at the renamer where "(>>)" is added to the
statement "ExprStmt" the first time, but apparently you cannot call
"tcSyntaxOp" in the typechecker on this function if you construct it
with "HsApp (compose_op `HsApp` then_op) guard_op". Is there another
function which could typecheck such a constructed expression without
telling the user what functions we've used?
I also had a look at the MDo typechecker, where they use this:
; let names = [mfixName, bindMName, thenMName,
returnMName, failMName]
; insts <- mapM (\name -> newMethodFromName DoOrigin name m_ty)
names
; return $ mkHsWrapCoI coi $
HsDo (MDoExpr (names `zip` insts)) stmts' body' res_ty' }
So they can use these names with their typechecked versions in the
desugarer. But if I do this for monad comprehensions and "guard",
*every* monad comprehension will require a MonadPlus instance, which
shouldn't be necessary if there are no filter expressions.
Any advice on how this could be solved?
Thanks, Nils
[1]: http://hackage.haskell.org/trac/ghc/ticket/4370
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