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The latest installment of "RyanGlScott finds bugs in Template Haskell pretty-printing". Here is what's featured on today's episode:

{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Bug where

import Data.Functor.Identity
import Language.Haskell.TH

$([d| f = \(Identity x) -> x |])
$([d| g = $(pure $ VarE '(+) `AppE` LitE (IntegerL (-1)) `AppE` (LitE (IntegerL (-1)))) |])

Running this with GHC 8.2 or later yields some incorrectly parenthesized output:

$ /opt/ghc/8.2.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
Bug.hs:8:3-31: Splicing declarations
    [d| f_azO = \ (Identity x_azP) -> x_azP |]
  ======>
    f_a3Nx = \ Identity x_a3Ny -> x_a3Ny
Bug.hs:9:3-90: Splicing declarations
    [d| g_a3NU
          = $(pure
                $ VarE '(+) `AppE` LitE (IntegerL (- 1))
                    `AppE` (LitE (IntegerL (- 1)))) |]
    pending(rn) [<splice_a3NV, pure
                                 $ VarE '(+) `AppE` LitE (IntegerL (- 1))
                                     `AppE` (LitE (IntegerL (- 1)))>]
  ======>
    g_a4dU = ((+) -1) -1

In particular, look at these two lines:

f_a3Nx = \ Identity x_a3Ny -> x_a3Ny
g_a4dU = ((+) -1) -1

These should be:

f_a3Nx = \ (Identity x_a3Ny) -> x_a3Ny
g_a4dU = ((+) (-1)) (-1)

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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