category feedHow to make code warning-freeMiscellaneouseditdelete

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What to do if you can't get rid of a warning

If you can't get rid of a warning, you can disable it explicitly in the module by adding a pragma. GHC prints flag names next to warnings:

lib/Fmt.hs:1189:1: warning: [-Worphans]
    Orphan instance:

So, to disable -Worphans, the pragma would have to look like this:

{-# OPTIONS_GHC -Wno-orphans #-}
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Starting from GHC 7.10, instead of {-# LANGUAGE OverlappingInstances #-} you have to mark specific instances with {-# OVERLAPPABLE #-}, {-# OVERLAPPING #-} or {-# OVERLAPS #-}. So, if you want to use overlapping instances, here's how you can do it:

#if __GLASGOW_HASKELL__ < 710
{-# LANGUAGE OverlappingInstances #-}
#  define _OVERLAPPING_
#  define _OVERLAPPABLE_
#  define _OVERLAPS_
#  define _OVERLAPS_ {-# OVERLAPS #-}

Now you can just mark your instances as instance _OVERLAPPING_ ... instead of instance {-# OVERLAPPING #-}, and it'll work without warnings on all GHC versions.

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Defined but not used
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Sometimes you have functions or types that you don't use anywhere in the module and don't export. In this case you will get these warnings:

test.hs:6:1-3: warning: [-Wunused-top-binds] …
    Defined but not used: ‘foo’

test.hs:11:1-14: warning: [-Wunused-top-binds] …
    Defined but not used: type constructor or class ‘Foo’

One way to silence them is to add {-# OPTIONS -Wno-unused-top-binds #-} to your module. Another is to add _ in front of all unused functions' names, like this:

_foo :: Int
_foo = 1

Unfortunately, this only works with functions and not types. Moreover, sometimes you can't modify function names as they are generated (by Template Haskell, for instance). In this case you can add a fake “usage” to the end of the file:

foo, bar :: Int
foo = 1
bar = 2
_unused :: ()
_unused = const () (foo, bar)

The same could be done for types:

type Foo = Int
type Bar = Bool
_unusedTypes :: (Foo, Bar)
_unusedTypes = undefined

(If you don't want to use undefined, you can use Proxy.)

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