[GHC] #5218: Add unpackCStringLen# to create Strings from string literals

[GHC]

#5218: Add unpackCStringLen# to create Strings from string literals
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        Reporter:  tibbe             |                Owner:  thoughtpolice
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.0.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #5877 #10064      |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Mathnerd314):

 I have yet another proposal, namely to add low-level construction methods
 to `IsString`:
 {{{#!hs
 class IsString a where
   fromString :: String -> a
   -- as before

   fromString_asc# :: Addr# -> Int# -> a
   fromString_asc# a _ = fromString (unpackCString# a)

   fromString_utf8# :: Addr# -> Int# -> a
   fromString_utf8# a _ = fromString (unpackCStringUtf8# a)

   fromString_raw# :: Addr# -> Int# -> a
   fromString_raw# a i = fromString (map chr (addrToWord8List a i)
 }}}

 The `String` instance would have `fromString = id`, while the `ByteString`
 instance would do zero-copy construction with the length information.
 ASCII literals would use _asc, UTF-8 literals would use _utf8, and
 primitive string literals would use _raw. Then `"x"# :: ByteString` would
 typecheck and work as expected.

 The only tricky part is preserving `"x"# :: Addr#` (which would be
 expanded to `fromString_raw# "x"# 1# :: Addr#`). It doesn't seem like
 `instance IsString Addr#` would be suitable.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5218#comment:39>
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