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Lists are always homogeneous, so you can't put items with a different <code>a</code> in to the same list (as you noticed). There are several ways to design around this, but I usually prefer using <a href="http://en.wikibooks.org/wiki/Haskell/GADT" rel="nofollow">GADTs</a>. For example: <pre><code>{-# LANGUAGE GADTs #-} import Data.ByteString (ByteString) import Data.Word data DuplicateReason = Size | Checksum | Content data DuplicateGroup data DuplicateTest where DuplicateTest :: Eq a => (FilePath -> IO a) -> DuplicateReason -> DuplicateTest getSize :: FilePath -> IO Integer getSize = undefined getChecksum :: FilePath -> IO Word32 getChecksum = undefined getContent :: FilePath -> IO ByteString getContent = undefined getDuplicatesByMethods :: [DuplicateTest] -> IO [DuplicateGroup] getDuplicatesByMethods = undefined </code></pre> This solution still needs a new type, but at least you don't have to specify all cases in advance or create boilerplate type-classes. Now, since the generic type <code>a</code> is essentially "hidden" inside the GADT, you can define a list that contains functions with different return types, wrapped in the <code>DuplicateTest</code> GADT. <pre><code>getDuplicatesByMethods [ DuplicateTest getSize Size , DuplicateTest getChecksum Checksum , DuplicateTest getContent Content ] </code></pre> You can also solve this without using any language extensions or introducing new types by simply re-thinking your functions. The main intention is to group files according to some property <code>a</code>, so we could define <code>getDuplicatesByMethods</code> as <pre><code>getDuplicatesByMethods :: [([FilePath] -> IO [[FilePath]], DuplicateReason)] -> IO [DuplicateGroup] </code></pre> I.e. we take in a function that groups files according to some criteria. Then we can define a helper function <pre><code>groupWith :: Eq a => (FilePath -> IO a) -> [FilePath] -> IO [[FilePath]] </code></pre> and call <code>getDuplicatesByMethods</code> like this <pre><code>getDuplicatesByMethods [ (groupWith getSize, Size) , (groupWith getChecksum, Checksum) , (groupWith getContent, Content) ] </code></pre>
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1. COI think just to get practice, I'll start from creating separate type (and typeclasses / helpers around that) to begin with that, and later check out GADTs. Thanks anyway.
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