Allow defining new functions with typed holes

[bradrn]

Is your feature request related to a problem? Please describe.

At the moment, the ‘Define <function>’ code action is hardcoded to produce name <> " = error \"not implemented\"" as the body of the function:

haskell-language-server/ghcide/src/Development/IDE/Plugin/CodeAction.hs

Lines 683 to 685 in 65a8512

	= [ ("Define " <> sig
	, [TextEdit (Range nextLineP nextLineP) (T.unlines ["", sig, name <> " = error \"not implemented\""])]
	)]

This is mostly satisfactory — but has the disadvantage of not producing a compiler error. If defining several new functions at once, it is easy to forget to go back and fill it in.

Describe the solution you'd like

It would be nice to have the option to define the function as a typed hole — so something like:

 = [ ("Define " <> sig 
     , [TextEdit (Range nextLineP nextLineP) (T.unlines ["", sig, name <> " = error \"not implemented\""])] 
+    )
+  , ("Define " <> sig <> " as typed hole"
+    , [TextEdit (Range nextLineP nextLineP) (T.unlines ["", sig, name <> " = _"])]
     )]

This would provide an easy way to reduce the number of code errors. It also provides a better UX: in my editor (Emacs), for instance, it would let me jump to and from the new function using flycheck-next-error etc.

[jneira]

thanks for the suggestion, coincidentally there is a pr which aims to implement it: #2465 (comment)

[bradrn]

Thanks @jneira! It’s a pretty minimal PR; any idea when it might be merged?

[jneira]

Afaics it "only" needs make the CI green 🙂

[michaelpj]

Fixed.