[commit: ghc] master: Suggest enabling PatternSynonyms (#10943) (1e8d1f1)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/1e8d1f1c6d85457c786b50b1e054facdb61cbae1/ghc

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

commit 1e8d1f1c6d85457c786b50b1e054facdb61cbae1
Author: Moritz Kiefer <moritz.kiefer at purelyfunctional.org>
Date:   Thu Oct 22 13:40:41 2015 +0200

    Suggest enabling PatternSynonyms (#10943)
    
    Suggest enabling PatternSynonyms if we find an invalid
    signature that looks like a pattern synonym.
    
    Reviewed By: austin, thomie
    
    Differential Revision: https://phabricator.haskell.org/D1347


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

1e8d1f1c6d85457c786b50b1e054facdb61cbae1
 compiler/parser/RdrHsSyn.hs                                | 14 +++++++++-----
 testsuite/tests/parser/should_fail/NoPatternSynonyms.hs    |  3 +++
 .../tests/parser/should_fail/NoPatternSynonyms.stderr      |  4 ++++
 testsuite/tests/parser/should_fail/all.T                   |  1 +
 4 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/compiler/parser/RdrHsSyn.hs b/compiler/parser/RdrHsSyn.hs
index e64a575..a1577a7 100644
--- a/compiler/parser/RdrHsSyn.hs
+++ b/compiler/parser/RdrHsSyn.hs
@@ -947,11 +947,14 @@ checkValSig lhs@(L l _) ty
                        ppr lhs <+> text "::" <+> ppr ty)
                    $$ text hint)
   where
-    hint = if foreign_RDR `looks_like` lhs
-           then "Perhaps you meant to use ForeignFunctionInterface?"
-           else if default_RDR `looks_like` lhs
-                then "Perhaps you meant to use DefaultSignatures?"
-                else "Should be of form <variable> :: <type>"
+    hint | foreign_RDR `looks_like` lhs =
+           "Perhaps you meant to use ForeignFunctionInterface?"
+         | default_RDR `looks_like` lhs =
+           "Perhaps you meant to use DefaultSignatures?"
+         | pattern_RDR `looks_like` lhs =
+           "Perhaps you meant to use PatternSynonyms?"
+         | otherwise =
+           "Should be of form <variable> :: <type>"
     -- A common error is to forget the ForeignFunctionInterface flag
     -- so check for that, and suggest.  cf Trac #3805
     -- Sadly 'foreign import' still barfs 'parse error' because 'import' is a keyword
@@ -961,6 +964,7 @@ checkValSig lhs@(L l _) ty
 
     foreign_RDR = mkUnqual varName (fsLit "foreign")
     default_RDR = mkUnqual varName (fsLit "default")
+    pattern_RDR = mkUnqual varName (fsLit "pattern")
 
 
 checkDoAndIfThenElse :: LHsExpr RdrName
diff --git a/testsuite/tests/parser/should_fail/NoPatternSynonyms.hs b/testsuite/tests/parser/should_fail/NoPatternSynonyms.hs
new file mode 100644
index 0000000..ef6f57f
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/NoPatternSynonyms.hs
@@ -0,0 +1,3 @@
+module NoPatternSynonyms where
+
+pattern P :: G Int
diff --git a/testsuite/tests/parser/should_fail/NoPatternSynonyms.stderr b/testsuite/tests/parser/should_fail/NoPatternSynonyms.stderr
new file mode 100644
index 0000000..b31ec83
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/NoPatternSynonyms.stderr
@@ -0,0 +1,4 @@
+
+NoPatternSynonyms.hs:3:1: error:
+    Invalid type signature: pattern P :: G Int
+    Perhaps you meant to use PatternSynonyms?
diff --git a/testsuite/tests/parser/should_fail/all.T b/testsuite/tests/parser/should_fail/all.T
index 13fb738..66945dd 100644
--- a/testsuite/tests/parser/should_fail/all.T
+++ b/testsuite/tests/parser/should_fail/all.T
@@ -68,6 +68,7 @@ test('T3811e', normal, compile_fail, [''])
 test('T3811f', normal, compile_fail, [''])
 test('T3811g', normal, compile_fail, [''])
 test('NoDoAndIfThenElse', normal, compile_fail, [''])
+test('NoPatternSynonyms', normal, compile_fail, [''])
 test('NondecreasingIndentationFail', normal, compile_fail, [''])
 test('readFailTraditionalRecords1', normal, compile_fail, [''])
 test('readFailTraditionalRecords2', normal, compile_fail, [''])