Can't Quote Instance Associated Types in Template Haskell

Give this a whirl:

{-# LANGUAGE TypeFamilies,TemplateHaskell #-}
module Bug where

    class C a where
        type T a

    $([d|  
        instance C (Maybe a) where
            type T (Maybe a) = Char
        |])

$ ghc -c Bug.hs 
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Loading package array-0.3.0.0 ... linking ... done.
Loading package containers-0.3.0.0 ... linking ... done.
Loading package pretty-1.0.1.1 ... linking ... done.
Loading package template-haskell ... linking ... done.

Bug.hs:7:6:
    Type indexes must match class instance head
    Found `Maybe a[aMy]' but expected `Maybe a[aMx]'
    In the associated type instance for `T'
    In the instance declaration for `C (Maybe a[aMx])'
[glastonbury:~/Projects/Haskell/Truth/Core]$ 

Trac metadata

Trac field	Value
Version	6.12.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug TemplateHaskell Trac import labels

Bit someone else, too: http://haskell.org/pipermail/glasgow-haskell-users/2010-January/018291.html

Workaround is to construct the instance in TH rather than use a quote.

Actually the email cited has a different problem

{-# LANGUAGE TemplateHaskell, MultiParamTypeClasses, TypeFamilies,
             FlexibleInstances, OverlappingInstances #-}

module Sample where

import Control.Monad
import Language.Haskell.TH

class Foo a where
    type FooType a

createInstance' :: Q Type -> Q Dec
createInstance' t = liftM head [d|
    instance Foo $t where
    type FooType $t = String |]

This fails with a similar error:

Sample.lhs:22:10:
    Type indexes must match class instance head
    Found `t_aMn' but expected `t_aMl'

Here it's plain that we can't really do full type-checking of the quoted instance until createInstance' is applied, so that we can run the splice $t. Somehow TH needs to be less picky about the consistency checks on types when typechecking quotes.

The earlier example, though, should be fine. I'll look into it.

Simon

Ah. My actual problem is closer to the email than to my example, because I am also splicing in the types.

OK. That will require a bit more thought. Use the workaround for now!

Simon

Just a brief whine: I encountered this problem, and I'm not even sure it's possible to apply the workaround to my task. I'm trying to use TH to generate a large number of very straightforward class methods, but there's also a single associated type which is too complicated to derive (and hellish to pass to my TH method).

Personally, I'd settle in an instant for being able to splice in declarations at the class method level, which currently gives a parse error.

mentioned in issue #4230 (closed)

See #4230 (closed) for a more global discussion

Simon

Trac metadata

Trac field	Value
BlockedBy	- → #4230 (closed)

Looks like this has bit me as well: http://stackoverflow.com/questions/8410761/using-template-haskell-how-can-i-splice-the-same-type-into-multiple-locations

changed milestone to %7.6.1

changed weight to 3

Trac metadata

Trac field	Value
Priority	normal → low

mentioned in commit 68217892

mentioned in commit ea2f35b5

mentioned in commit f2580924

changed milestone to %7.6.2

Why is this still open? Above commit log says that commit fixes this bug, and test case is already added by [changeset:ccd14ae24656440e4746a888d070b8922871408a/testsuite].

Trac metadata

Trac field	Value
Test case	→ th/T4135

shelarcy: The program from ticket:4135#comment:41694 still does not compile in HEAD.

Replying to [ticket:4135#comment:75827 monoidal]: I see. I missed that.

mentioned in commit f6f9eb36

closed

Comment 2 works now, with the new Template Haskell story. I've added a new test T4135a.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	th/T4135 → th/T4135, T4135a

added Plow label