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

[Michael Baikov]

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

Thanks!

> 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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