[GHC] #4168: Add StringPrimL to template-haskell

[GHC]

#4168: Add StringPrimL to template-haskell
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    Reporter:  ekmett            |       Owner:              
        Type:  feature request   |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  6.12.3            |    Keywords:              
          Os:  Unknown/Multiple  |    Testcase:              
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
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 In designing a template haskell lexer, you cannot get the same performance
 as alex, due to the fact that you cannot encode a `"string"#` literal to
 house the lookup tables for the encoded lexer.

 Proposal: add `"string"#` literals to `template-haskell` as `StringPrimL`
 (or another suitable constructor) to Lit.

 Then the quasiquoter can emit a data type that contains `Addr#` references
 to the link-time-locatable `"string"#`s permitting constant folding and
 inlining away the particular link time choice of `Addr#`.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4168>
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