[Haskell-cafe] Template haskell: This pattern-binding binds no variables

21 Dec 2014

      I'm trying to introduce a new local variable to some scope, in this
example this scope is represented by print $( varE ohNoes)

Everything works as expected, but I'm getting a warning message which
I don't seem quite right to me. Am I doing anything wrong?

---- Blah.hs ----
{-# LANGUAGE TemplateHaskell #-}

module Blah where

import Language.Haskell.TH

sample :: Q Exp
sample = newName "ohNoes" >>=  \ohNoes -> [| let $( varP ohNoes ) =
"Teh warning!" in print $( varE ohNoes ) |]

---- Main.hs ----
{-# LANGUAGE TemplateHaskell #-}

module Main where
import Blah

main :: IO ()
main = $( sample )

---- output ----

Blah.hs:8:49: Warning:
    This pattern-binding binds no variables:
      $(varP ohNoes) = "Teh warning!"

    sample
  ======>
    let ohNoes_a2Al = "Teh warning!" in print ohNoes_a2Al

[Haskell-cafe] Template haskell: This pattern-binding binds no variables

Michael Baikov