[Haskell-cafe] Display an inferred type during compilation

Ilya Portnov portnov at iportnov.ru
Sat Apr 27 20:46:00 CEST 2013

В письме от 27 апреля 2013 18:55:16 пользователь Corentin Dupont написал:

Hi Cafe,can I ask the compiler to display the type of an inferred value during compile 
time?It would be great if I can output a string during compilation with the type.A little 
bit like running :type in GHCi, but without GHCi... Because running GHCi is sometime 
painful (I have to clean my code first).

I'm thinking of something like:

main :: IO ()main = do   a <- someCode   displayTypeAtCompileTime a   return ()

$ ghc -c test.hstest.hs:4:3: your type is: Foo



What about TemplateHaskell? Smth like:

{-# LANGUAGE TemplateHaskell #-}
module DisplayType where

import Language.TH

displayTypeAtCompileTime :: Name -> Q Exp
displayTypeAtComileTime name = do
    reified <- reify name
      -- inspect reified structure, see TH haddock documentation
    runIO $ putStrLn $ show theType
     [| undefined |] -- you need to return some expression; since you are not to use it's 
value, it may be even undefined, it seems.


{-# LANGUAGE TemplateHaskell #-}
module Main where

import DisplayType

main = do
    $displayTypeAtCompileTime 'a

WBR, Ilya Portnov.

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