#2221: Can't use quotationes ([| ... |]) insides declaration splices
-----------------------+----------------------------------------------------
Reporter: m4dc4p | Owner:
Type: bug | Status: new
Priority: normal | Component: Template Haskell
Version: 6.8.2 | Severity: normal
Keywords: | Testcase:
Architecture: x86 | Os: Windows
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Normal Template Haskell allows many levels of splicing and quotations:
{{{
> runQ [| (const $([|"Int"|])) |]
AppE (VarE GHC.Base.const) (LitE (StringL "Int"))
}}}
And TH can parse declarations easily:
{{{
> runQ [d| type T = Int|]
[TySynD T [] (ConT GHC.Base.Int)]
}}}
But splicing inside a declaration gives a syntax error:
{{{
> runQ [d| type T = $([t|Int|])|]
parse error on input `$('
}}}
Another example. Bulat Ziganshin defines the cnst function
(http://www.haskell.org/bz/th3.htm) as:
{{{
cnst 0 str = [| str |]
cnst n str = [| \_ -> $(cnst (n-1) str) |]
}}}
Which evaluates to a function that takes n arguments and returns the
string given. It would make sense that a function to define a data
constructor which takes n arguments could be written similarly:
{{{
dataCnst n = [d|data D = D $(dataCnst' n)|]
dataCnst' n = [t|Int|] ++ dataCnst' (n - 1)
}}}
Finally, a function that makes a data type which varies the field type
based on an argument:
{{{
dataVar n = [d|data D = D $(dataVar' n)|]
dataVar' "Int" = ''Int
dataVar' "String" = ''String
etc.
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2221>
GHC <http://www.haskell.org/ghc/>
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