[commit: ghc] master: OccName: Implement startsWithUnderscore in terms of headFS (fc53d36)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/fc53d3658ad87073fbdc080f05ad3ac01d1ec59a/ghc

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

commit fc53d3658ad87073fbdc080f05ad3ac01d1ec59a
Author: Ben Gamari <bgamari.foss at gmail.com>
Date:   Fri Jul 8 14:09:36 2016 +0200

    OccName: Implement startsWithUnderscore in terms of headFS
    
    This avoids decoding the entire string just to look at the first
    character.
    
    Test Plan: Validate
    
    Reviewers: austin
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D2388


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fc53d3658ad87073fbdc080f05ad3ac01d1ec59a
 compiler/basicTypes/OccName.hs | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/compiler/basicTypes/OccName.hs b/compiler/basicTypes/OccName.hs
index 6a5c489..caaf90b 100644
--- a/compiler/basicTypes/OccName.hs
+++ b/compiler/basicTypes/OccName.hs
@@ -521,9 +521,7 @@ parenSymOcc occ doc | isSymOcc occ = parens doc
 startsWithUnderscore :: OccName -> Bool
 -- ^ Haskell 98 encourages compilers to suppress warnings about unsed
 -- names in a pattern if they start with @_@: this implements that test
-startsWithUnderscore occ = case occNameString occ of
-                             ('_' : _) -> True
-                             _other    -> False
+startsWithUnderscore occ = headFS (occNameFS occ) == '_'
 
 {-
 ************************************************************************