Variables in patterns made with QuasiQuotes sometimes don't get bound

In GHCi 8.0.2

>>> import Language.Haskell.TH
>>> import Language.Haskell.TH.Quote
>>> :set -XQuasiQuotes
>>> quoter = QuasiQuoter { quotePat = varP . mkName }
>>> [quoter|x|] = 1
>>> x
error: Variable not in scope: x

I think "x" ought to be 1.

Previous versions of GHCi don't let me be so free about defining and using quasiquotes, so here is a minimal example with modules:

-- In QQ.hs
module QQ where

import Language.Haskell.TH
import Language.Haskell.TH.Quote

quoter :: QuasiQuoter
quoter = QuasiQuoter { quotePat = varP . mkName }

and

-- In Main1.hs
module Main1 where

import QQ

[quoter|x|] = 1

main = print x

With GHC 7.10.3 this compiles (and prints "1" when run), but in GHC 8.0.2 this complains "x" is not in scope. The same problem manifests itself when the pattern quasi quote is used in a "let" binding.

Peculiarly, this appears not to affect cases where the pattern is a function argument. The following compiles and runs (printing "1") on both 7.10.3 and 8.0.2.

-- In Main2.hs
module Main2 where

import QQ

f [quoter|x|] = x

main = print (f 1)

I understand there were some changes around splices and declaration groups (which presumably are part of why GHCi plays with quasi quotes), so I'm not sure this is really a bug!

Trac metadata

Trac field	Value
Version	8.0.2
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

Actually, it looks like this is equivalent to a more general TH issue (with a smaller minimal example). The following compiles in 7.10.3 but not 8.0.2

{-# LANGUAGE TemplateHaskell #-}
module Main where

import Language.Haskell.TH

main = let $(varP $ mkName "x") = 1 in print x

This regression appeared between GHC 8.0.1 and 8.0.2. I tracked it down to commit 8d634194:

From 8d63419478074728eb03082787ea51d498b3e62e Mon Sep 17 00:00:00 2001
From: =?utf8?q?Facundo=20Dom=C3=ADnguez?= <facundo.dominguez@tweag.io>
Date: Wed, 6 Jul 2016 06:48:27 -0300
Subject: [PATCH] Have addModFinalizer expose the local type environment.

Summary:
This annotates the splice point with 'HsSpliced ref e' where 'e' is the
result of the splice. 'ref' is a reference that the typechecker will fill with
the local type environment.

The finalizer then reads the ref and uses the local type environment, which
causes 'reify' to find local variables when run in the finalizer.

Thoughts, Facundo?

Trac metadata

Trac field	Value
Component	Compiler → Template Haskell

Oops, I cc'd the wrong username, it seems.

Trac metadata

Trac field	Value
CC	facundominguez → facundo.dominguez

Trac metadata

Trac field	Value
Differential revisions	- → D3572

mentioned in commit eaf9cc42

Thanks, Ryan. Please, excuse me for not looking at this earlier.

changed milestone to %8.2.1

No worries, Facundo!

I'll optimistically mark this is merge, since it seems to be a straightforward bugfix. Feel free to close if you think otherwise.

added 1 deleted label

closed

Merged with 72015aba.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

added Pnormal label