• simonpj@microsoft.com's avatar
    Add quasi-quotation, courtesy of Geoffrey Mainland · f3399c44
    simonpj@microsoft.com authored
    This patch adds quasi-quotation, as described in
      "Nice to be Quoted: Quasiquoting for Haskell"
    	(Geoffrey Mainland, Haskell Workshop 2007)
    Implemented by Geoffrey and polished by Simon.
    The syntax for quasiquotation is very similar to the existing
    Template haskell syntax:
    	[$q| stuff |]
    where 'q' is the "quoter".  This syntax differs from the paper, by using
    a '$' rather than ':', to avoid clashing with parallel array comprehensions.
    The "quoter" is a value of type Language.Haskell.TH.Quote.QuasiQuoter, which
    contains two functions for quoting expressions and patterns, respectively.
         quote = Language.Haskell.TH.Quote.QuasiQuoter quoteExp quotePat
         quoteExp :: String -> Language.Haskell.TH.ExpQ
         quotePat :: String -> Language.Haskell.TH.PatQ
    TEXT is passed unmodified to the quoter. The context of the
    quasiquotation statement determines which of the two quoters is
    called: if the quasiquotation occurs in an expression context,
    quoteExp is called, and if it occurs in a pattern context, quotePat
    is called.
    The result of running the quoter on its arguments is spliced into
    the program using Template Haskell's existing mechanisms for
    splicing in code. Note that although Template Haskell does not
    support pattern brackets, with this patch binding occurrences of
    variables in patterns are supported. Quoters must also obey the same
    stage restrictions as Template Haskell; in particular, in this
    example quote may not be defined in the module where it is used as a
    quasiquoter, but must be imported from another module.
    Points to notice
    * The whole thing is enabled with the flag -XQuasiQuotes
    * There is an accompanying patch to the template-haskell library. This
      involves one interface change:
    	currentModule :: Q String
      is replaced by
    	location :: Q Loc
      where Loc is a data type defined in TH.Syntax thus:
          data Loc
            = Loc { loc_filename :: String
    	      , loc_package  :: String
    	      , loc_module   :: String
    	      , loc_start    :: CharPos
    	      , loc_end      :: CharPos }
          type CharPos = (Int, Int)	-- Line and character position
      So you get a lot more info from 'location' than from 'currentModule'.
      The location you get is the location of the splice.
      This works in Template Haskell too of course, and lets a TH program
      generate much better error messages.
    * There's also a new module in the template-haskell package called 
      Language.Haskell.TH.Quote, which contains support code for the
      quasi-quoting feature.
    * Quasi-quote splices are run *in the renamer* because they can build 
      *patterns* and hence the renamer needs to see the output of running the
      splice.  This involved a bit of rejigging in the renamer, especially
      concerning the reporting of duplicate or shadowed names.
      (In fact I found and removed a few calls to checkDupNames in RnSource 
      that are redundant, becuase top-level duplicate decls are handled in
RnPat.lhs 25.1 KB