[GHC] #10047: inconsistency in name binding between splice and quasiquotation

30 Jan 2015

      #10047: inconsistency in name binding between splice and quasiquotation
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              Reporter:  rwbarton    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Template    |           Version:  7.8.4
  Haskell                            |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 Let me preface this by saying that this may not be a bug. If not then it
 would be nice if the documentation for Template Haskell could clarify
 what's going on here.

 My understanding of quasiquotation is that a quasiquote `[n|foo|]` is
 equivalent to a splice `$(quoteExp n "foo")`. However, that is not the
 case in all contexts.

 {{{
 module Q where

 import Language.Haskell.TH
 import Language.Haskell.TH.Quote

 n = QuasiQuoter { quoteExp = dyn }
 }}}

 {{{
 rwbarton@morphism:/tmp$ ghci -XTemplateHaskell -XQuasiQuotes Q
 GHCi, version 7.8.3: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling Q                ( Q.hs, interpreted )

 Q.hs:6:5: Warning:
     Fields of ‘QuasiQuoter’ not initialised: quotePat, quoteType,
                                              quoteDec
     In the expression: QuasiQuoter {quoteExp = dyn}
     In an equation for ‘n’: n = QuasiQuoter {quoteExp = dyn}
 Ok, modules loaded: Q.
 *Q> :t [| $(dyn "foo") |]
 [| $(dyn "foo") |] :: ExpQ
 *Q> :t [| [n|foo|] |]
 Loading package pretty-1.1.1.1 ... linking ... done.
 Loading package array-0.5.0.0 ... linking ... done.
 Loading package deepseq-1.3.0.2 ... linking ... done.
 Loading package containers-0.5.5.1 ... linking ... done.
 Loading package template-haskell ... linking ... done.

 <interactive>:1:7:
     Not in scope: ‘foo’
     In the Template Haskell quotation [| [n|foo|] |]
 }}}

 Why do these behave differently?

 (Lastly, the link to the paper "Why It's Nice to be Quoted: Quasiquoting
 for Haskell" in the User's Guide at
 https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/template-
 haskell.html#th-quasiquotation is broken. Does this paper have a permanent
 home? In any case, I only skimmed it but it didn't seem to answer my
 question.)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10047
GHC http://www.haskell.org/ghc/
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