Print and execute a string

Question

I find myself writing a lot of code like

putStr "foo (bar 1) (bar 2) ="
print $ foo (bar 1) (bar 2)

The trouble is, the printed message can get out of sync with the actual executed code. The obvious solution is to auto-generate this code.

One way to do that would be to put all the text in a file, and write a small program that reads the file and generates Haskell source code from it. But another alternative is to use Template Haskell.

Does anybody know how I would go about writing a function that takes a String and generates the above code from it? I'm guessing it should be pretty easy, but TH is not well documented.

Answer 1

You can parse Haskell code using the haskell-src-meta package. Here's a quick example how you could combine this with Template Haskell.

{-# LANGUAGE TemplateHaskell #-}

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

runShow = QuasiQuoter
    { quoteExp  = runShowQQ
    , quotePat  = undefined
    , quoteType = undefined
    , quoteDec  = undefined
    }

runShowQQ :: String -> Q Exp
runShowQQ s = do
    let s'          = s ++ " = "
        Right exp = parseExp s
        printExp  = appE [|print|] (return exp)
    infixApp [|putStr s'|] [|(>>)|] printExp

And you would use it like this

{-# LANGUAGE QuasiQuotes #-}

[runShow|foo (bar 1) (bar 2)|]

Answer 2

Template Haskell does not provide a straightforward means of parsing arbitrary strings, so the simplest solution is probably to use the C preprocessor. However, the built-in one in GHC does not support stringification, so we need to pass extra options to use the "real" one instead.

{-# LANGUAGE CPP #-}
{-# OPTIONS_GHC -pgmP cpp #-}

#define PRINT_EXP(x) (putStr #x >> putStr " = " >> print (x))

You can then use it like this:

PRINT_EXP(foo (bar 1) (bar 2))