-XRebindableSyntax needs return?

The documentation for -XRebindableSyntax ( https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#rebindable-syntax-and-the-implicit-prelude-import ) reads:

“Do” notation is translated using whatever functions (>>=), (>>), and fail, are in scope (not the Prelude versions). List comprehensions, mdo (The recursive do-notation), and parallel array comprehensions, are unaffected.

The following code fails to compile (also confirmed to fail on GHC 8.0.2):

{-# LANGUAGE RebindableSyntax #-}

module Lib where

import Prelude (IO)

foo :: IO ()
foo = do
  pure ()

It fails with this error:

/home/kel317/z/src/Lib.hs:9:3: error:
    Not in scope: ‘return’
    Perhaps you want to add ‘return’ to the import list
    in the import of ‘Prelude’ (src/Lib.hs:5:1-19).
  |
9 |   pure ()
  |   ^^^^^^^

To my reading, either something funky is going on with the way the do-notation is desugared, or the documentation needs to be corrected.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

changed milestone to %8.6.1

Indeed that is quite curious. My first thought was perhaps ApplicativeDo was involved, but it's not enabled.

Trac metadata

Trac field	Value
Test case	- → rebindable/T14670

I see that in compiler/rename/RnExpr.hs there's a "TODO: I don't know how to get this right for rebindable syntax", but I don't think any of that is supposed to be used without ApplicativeDo. I haven't yet been able to find a place where ApplicativeDo checking obviously leaks out elsewhere.

mentioned in commit 765ba657

assigned to @simonmar

Simon: this is your bailiwick.

changed milestone to %8.8.1

These won't be addressed by GHC 8.6.

changed milestone to %8.10.1

Bumping milestones of low-priority tickets.

closed

This is fixed. With the latest GHC I get:

Lib.hs:9:3: error:
    • Variable not in scope: pure :: () -> IO ()
    • Perhaps you want to add ‘pure’ to the import list
      in the import of ‘Prelude’ (Lib.hs:5:1-19).
  |
9 |   pure ()
  |   ^^^^

I'm closing this ticket. Reopen if you disagree.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedWorksForMe

This still fails for me on GHC 8.6.3, which is the latest I see on https://www.haskell.org/ghc . If I've missed a later version, or you've confirmed a fix on GHC HEAD, then please re-close:

$ ghci Test.hs 
GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Test             ( Test.hs, interpreted )

Test.hs:7:3: error:
    Not in scope: ‘return’
    Perhaps you want to add ‘return’ to the import list
    in the import of ‘Prelude’ (Test.hs:4:1-19).
  
7 |   pure ()
  |   ^^^^^^^
Failed, no modules loaded.
Prelude> 

$ cat Test.hs
{-# LANGUAGE RebindableSyntax #-}

module Test where
import Prelude (IO)
foo :: IO ()
foo = do
  pure ()

reopened

Trac metadata

Trac field	Value
Resolution	ResolvedWorksForMe → Unresolved

closed

jackkelly, when josef wrote “latest GHC”, he probably meant HEAD, not the latest released version. This really works as expected with HEAD.

It would be interesting to find out which commit fixed it. I tried to git bisect but submodules don't cooperate with me (unsurprisingly). I got:

fatal: repository '../hadrian.git' does not exist

I tried various combinations of make maintainer-clean / boot-configure / git submodule update --recursive.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added RebindableSyntax label

added broken test label

added Pnormal label