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

[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

On Sun, Dec 21, 2014 at 3:50 AM, Michael Baikov <manpacket at gmail.com> wrote:
> 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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