Add change type signature quick fix

[jneira]

When you change the body of a function and the type signature does not match you get the obvious compiler error:

F.e. for

myfunc :: String
myfunc = forM

you get

• Couldn't match type ‘t0 a0 -> (a0 -> m0 b0) -> m0 (t0 b0)’
                 with ‘[Char]’
  Expected type: String
    Actual type: t0 a0 -> (a0 -> m0 b0) -> m0 (t0 b0)
• Probable cause: ‘forM’ is applied to too few arguments
  In the expression: forM
  In an equation for ‘myfunc’: myfunc = forM

It would be nice to have a quick fix (better than a code lens imo) which changes the type signature to match the body new type, something like Change type signature to: (Traversable t, Monad m) =>t a -> (a -> m b) -> m (t b)

That type is already shown on hover.

Additional context

• We already have a code lens to add the code signature, so this feature could reuse a lot from that one
• That feature is common in other ides like eclipse or intellij

[ncfavier]

Would love to see this as well: code lenses are distracting.

[michaelpj]

Note that there's already a code action for adding a missing signature, if you don't like the lens.

[ncfavier]

I couldn't find it. In Visual Studio Code, when clicking on the light bulb that appears next to a top-level definition, I only see retrie's fold/unfold actions.

[michaelpj]

Works for me! Unless lsp-mode is doing something weird and listing code lenses as code actions also?

[jneira]

Works for me! Unless lsp-mode is doing something weird and listing code lenses as code actions also?

I think that is the case, no code action in vscode for me neither

[jneira]

I would say code lenses for missing type signature is one of the more appropiate uses of lenses. It displays just the code which will be added if you click it, no replaces or subsitutions and helps you even if you dont add them.
But i agree even for missing type signatures it can be invasive and should have an alternative in the server itself.

@michaelpj what do you think to move upstream the code lenses as code actions thing of emacs lsp mode. Unconditionally or only if user disable the code lenses explicitly.

[michaelpj]

Isn't this defining a command? https://github.com/haskell/haskell-language-server/blob/master/ghcide/src/Development/IDE/Plugin/TypeLenses.hs#L78

[michaelpj]

Hmm, maybe that's only used to apply the edit, ignore me.

[michaelpj]

I can't find where lsp-mode does this, but I guess it must be.