[commit: ghc] master: Fix TH ppr output for list comprehensions with only one Stmt (0e7ccf6)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/0e7ccf6d233c66b23a60de4e35e039f78ea3e162/ghc

>---------------------------------------------------------------

commit 0e7ccf6d233c66b23a60de4e35e039f78ea3e162
Author: Ryan Scott <ryan.gl.scott at gmail.com>
Date:   Sun Sep 11 11:10:36 2016 -0400

    Fix TH ppr output for list comprehensions with only one Stmt
    
    A folow-up to D2521 (which addressed #12583), where the `Outputable` `ppr`
    output was tweaked to display a list comprehension with only one `Stmt` as
    `[Foo]` instead of `[Foo|]` (which isn't valid Haskell). I forgot to update
    the corresponding code in `Language.Haskell.TH.Ppr` which pretty-prints
    `CompE`, however, so this commit takes care of that.


>---------------------------------------------------------------

0e7ccf6d233c66b23a60de4e35e039f78ea3e162
 libraries/template-haskell/Language/Haskell/TH/Ppr.hs | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
index 0462a8d..7376135 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
@@ -179,12 +179,17 @@ pprExp i (DoE ss_) = parensIf (i > noPrec) $ text "do" <+> pprStms ss_
 
 pprExp _ (CompE []) = text "<<Empty CompExp>>"
 -- This will probably break with fixity declarations - would need a ';'
-pprExp _ (CompE ss) = text "[" <> ppr s
-                  <+> bar
-                  <+> commaSep ss'
-                   <> text "]"
-    where s = last ss
-          ss' = init ss
+pprExp _ (CompE ss) =
+    if null ss'
+       -- If there are no statements in a list comprehension besides the last
+       -- one, we simply treat it like a normal list.
+       then text "[" <> ppr s <> text "]"
+       else text "[" <> ppr s
+        <+> bar
+        <+> commaSep ss'
+         <> text "]"
+  where s = last ss
+        ss' = init ss
 pprExp _ (ArithSeqE d) = ppr d
 pprExp _ (ListE es) = brackets (commaSep es)
 pprExp i (SigE e t) = parensIf (i > noPrec) $ ppr e <+> dcolon <+> ppr t