Quasi quoting

Max Bolingbroke batterseapower at hotmail.com
Tue Feb 2 19:48:27 EST 2010

2010/2/2 Twan van Laarhoven <twanvl at gmail.com>:
>    class Quoted a where
>        parseQuote :: String -> a
>        -- for performance reasons:
>        parseQuote' :: Ghc.PackedString -> a

Great idea!

Thinking about it, you can use type classes to dispose of the
QuasiQuote record entirely. Instead, have:

class MyLang a where
  myLang :: String -> Q a

instance MyLang Exp where
  myLang = myLangSyntaxToGHCExprForSplice . myLangExpParser

... etc, MyLang instances for Pat and Type too ...

And then write:

$(myLang [|...|])

Now the splice $(e) typechecks e as a Q Type / Q Exp / Q Decl as
required by the context it is in, and hence gets the correct instance
of MyLang. So our proposal needn't change the semantics of splice at
all - we can reuse the name overloading abilities of type classes.

It's a shame that TH is too widely used to be amenable to refactoring
into this sort of form.


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