#4370: Bring back monad comprehensions
---------------------------------+------------------------------------------
Reporter: simonpj | Owner: nsch
Type: feature request | Status: new
Priority: normal | Milestone: 7.2.1
Component: Compiler | Version: 6.12.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by nsch):
I have a more technical question about the core language generation in the
desugarer. I'm trying to use a function from the GHC.Exts library to
simplify
core generation a bit and since I only lookup the functions `Id` I have to
manually apply all the types etc. to the function. The code looks
something
like this:
{{{
do { [..]
; mmap_id <- dsLookupGlobalId mmapName -- new function from GHC.Exts
; let -- Apply types & arguments to 'mmap'
tupleElem n = mkApps (Var mmap_id)
-- Types:
-- mmap :: forall (m :: * -> *) a b. Monad
m => ..
[ Type m_ty, Type a_ty, Type b_ty
-- Arguments:
, .. ]
; [..] }
}}}
But that expression is missing one more type argument for that `Monad m
=>`
predicate, which leads to the following error (for a simple example with
lists
and integers):
{{{
C:\Users\Nils\dev\hiwi\ghc\inplace\bin>ghc-stage2.exe -dcore-lint
mc_group.hs
[2 of 2] Compiling Main ( mc_group.hs, mc_group.o )
*** Core Lint errors : in result of Desugar ***
<no location info>:
In the expression: GHC.Exts.mmap
@ []
@ (GHC.Types.Int, GHC.Types.Int)
@ GHC.Types.Int
(\ (ds_dqc :: (GHC.Types.Int, GHC.Types.Int)) ->
case ds_dqc of _ { (ds_dq9, _) -> ds_dq9 })
Argument value doesn't match argument type:
Fun type:
GHC.Base.Monad [] =>
((GHC.Types.Int, GHC.Types.Int) -> GHC.Types.Int)
-> [(GHC.Types.Int, GHC.Types.Int)]
-> [GHC.Types.Int]
Arg type: (GHC.Types.Int, GHC.Types.Int) -> GHC.Types.Int
Arg:
\ (ds_dqc :: (GHC.Types.Int, GHC.Types.Int)) ->
case ds_dqc of _ { (ds_dq9, _) -> ds_dq9 }
}}}
Comparing this core dump with other core dumps, it's clear that there
should be
another argument `GHC.Base.$fMonad[]` (for this example) to the `mmap`
function, but I don't know what kind of argument that is and how to get
it. So
my question is pretty simple: What function will get me that missing
argument?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4370#comment:28>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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