Why do my top level functions need signatures in Haskell?

Question

GHC warns that I don't have the function signatures at the top level. I don't get why I would need them. The problem with providing them is that they are quite sophisticated, like this one (autogenerated):

applyValue :: forall t t1 t2 t3 t4.
                (t2 -> t)
                -> (t2 -> t3 -> t4 -> t1) -> t2 -> t3 -> t4 -> (t -> Bool) -> [t1]

So why would I bother adding them?

the function itself:

applyValue getValueAt stitchAndMove at fabric mark matchAt =
   if matchAt (getValueAt at)
   then [stitchAndMove at fabric mark]
   else []

If this happens because you're compiling with -Wall, you can use -fno-warn-missing-signatures. It's considered good practice not to leave them out - see Daniel Wagner's answer for reasons why. — Mikhail Glushenkov
– Mikhail Glushenkov, Commented Oct 28, 2013 at 3:38

Daniel Wagner · Accepted Answer · 2013-10-28 04:04:23Z

25

As a form of machine-checkable documentation. If you believe that type to be the right type, putting it there asks the compiler to double-check that you didn't hose your own interface during your later inevitable refactoring sessions.
As human-readable documentation. Although as you observe, when you notice you're writing an awful machine-generated type, it's probably time to think about what (type-level) abstractions you need to make it human-readable.
For haddock. Haddock comments get attached to type signatures, not bindings, so if you leave out a type signature, your carefully hand-written documentation will be silently ignored.
To improve error messages and ghci query results: although the actual names of type variables don't matter, GHC tries hard to preserve names when they're provided by the user. Something like (node -> Bool) -> (edge -> Bool) -> (graph -> Bool) can be much more readable than (t1 -> Bool) -> (t2 -> Bool) -> (t3 -> Bool), even though they're equivalent.

edited Oct 28, 2013 at 4:04

answered Oct 28, 2013 at 3:39

Daniel Wagner

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3 Comments

Trident D'Gao Over a year ago

Your point makes sense, I have another quick question though, is there a way to reuse signature blocks, say I have a signature of a function f1 that is long and sophisticated, now f2 takes f1, so it's everything about f2 + whatever f1 was. I wonder if I can save some work by assigning the signature f1 to some of name (type?) and refer to it by that name when I am defining the signature of f2. Thanks

Daniel Wagner Over a year ago

@AlekseyBykov Yes, this is what type aliases do. You can read about them in the Report or your favorite tutorial.

newacct Over a year ago

Signatures are sometimes necessary even if you don't care about documentation or error messages. Sometimes it is ambiguous what instance of a type class you want, and thus you need to explicitly constrain it in order for it to compile.

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