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

[Michael Baikov]

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
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[adam vogt]

Hi Michael,

I think it's a bug. Maybe the fix is similar to the one done for
https://ghc.haskell.org/trac/ghc/ticket/9127.

Assuming you only need that unique name ('ohNoes :: Name) after you
bind it with the let, you can avoid the warning with:

sample :: Q Exp
sample = [| let ohNoes = "Teh warning!"

in print $( varE 'ohNoes ) |]

Or you could use letE instead of the brackets, as we had to with ghc <
7.8 when PatQ splices were added.

Regards,
Adam

[Michael Baikov]

I need this unique iname in other places as well. letE would work,
right. I'll file a bugreport later then,

Thanks!