Can a quasiquoter 'use' variables defined in the same file it is called?

Question

So, I'm starting to experiment with quasiquotation and template haskell.

I want to modify an existing (large) quasiquotation code, while using the actual value of a variable defined where it is 'called'. To illustrate with a simple example:

main.hs

{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH
import Exp02

x = "cde"

main = do
  putStrLn [str|$x|]

Exp02.hs

{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}

module Exp02 where

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

xpto :: String -> ExpQ
xpto [] = stringE []
xpto ('$':rest) = varE (mkName rest)
xpto str = stringE str

str = QuasiQuoter
  { quoteExp = xpto
  , quotePat = fail $ "patterns"
  , quoteType= fail $ "types"
  , quoteDec = fail $ "declarations"
  }

While this compiles and prints out "cde", this is not what I want. My understanding is that the resulting code in main after splicing is: putStrLn x. What I want is to generate putStrLn cde (I know this is not valid haskell code, but it's just to represent my point).

Thus, to put it in another way, I do not want to 'create a reference to the variable x in the main file', I want to actually use its value inside the xpto quasiquoter code.

I am guessing this may not be possible, since it would imply a circular reference between main.hs and Exp02.hs, and thus face the TH stage restriction. Is this correct, or is there a way to use x value inside the xpto code?

Thanks!

score 6 · Accepted Answer · edited Jun 20 '20 at 09:12

No, what you are trying to do isn't currently possible. From the template haskell docs:

You can only run a function at compile time if it is imported from another module that is not part of a mutually-recursive group of modules that includes the module currently being compiled. Furthermore, all of the modules of the mutually-recursive group must be reachable by non-SOURCE imports from the module where the splice is to be run.

For example, when compiling module A, you can only run Template Haskell functions imported from B if B does not import A (directly or indirectly). The reason should be clear: to run B we must compile and run A, but we are currently type-checking A.

You are attempting to run the function (or more strictly value) x at compile time in the same module as x was defined, which is clearly stated to not be allowed.

Can a quasiquoter 'use' variables defined in the same file it is called?

1 Answers1