Quoting functions in Template Haskell

September 7, 2015

While working on this patch, I learned a cool feature of Template Haskell called quoting function. Instead of using VarE (mkName "show") to create an Exp type, you can safely create the same using this: VarE 'Prelude.show. When generating declartions using splicing, the import of the module will be taken care of by itself. A self contained example:

{-#LANGUAGE TemplateHaskell#-}
-- External.hs
module External where

import Data.Time
import Language.Haskell.TH

example :: Name
example = 'getCurrentTime

sampleDec :: Q [Dec]
sampleDec = return $
[ValD (VarP (mkName "hello")) (NormalB (VarE example)) []]

And the other module:

{-#LANGUAGE TemplateHaskell#-}
-- Other.hs
import External


main = do
  x <- hello
  print x

Now you can see this in ghci:

λ> main
2015-09-01 22:01:34.532374 UTC

See how I didn’t have to import Data.Time in the Other.hs file. The single quote can be also used for data constructors. Similarly, a type can be converted to Name using double quote.