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 []

Answer 1

• 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.