[Haskell-cafe] Unused Arrow commands do not give warning

Tom Ellis tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk
Wed Nov 6 14:17:48 UTC 2013

GHC can give you a warning when you fail to use the result of a monadic
action in a do block.  The same does not hold for arrow commands in arrow
notation.  (See demonstration below signature).

What would it take to add a warning for this?  I am writing arrow-heavy code
at the moment and I think such a warning would be a nice feature.


% cat Test.hs
{-# LANGUAGE Arrows #-}

module Test where

import Control.Arrow (returnA, arr)
import Control.Monad.Identity (Identity(..))

foo :: Identity ()
foo = do
    Identity True
    return ()

bar :: () -> ()
bar = proc () -> do
    arr (const True) -< ()
    returnA -< ()
% ghc -fforce-recomp -Wall Test.hs
[1 of 1] Compiling Test             ( Test.hs, Test.o )

    Warning: A do-notation statement discarded a result of type Bool.
             Suppress this warning by saying "_ <-
             or by using the flag -fno-warn-unused-do-bind

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