One feature I've always wanted would be able to "manually inline" a function. For example, if I have
foo a = a + a
bar = foo 3
I'd like to be able to right-click on the invocation of `foo` and select some option to inline it, rewriting the file to read
foo a = a + a
bar = 3 + 3
I think this would make it simple to dive into an abstraction to see what's really going on. For example, imagine a new user is confused by this expression:
Just 3 >> Just 4
They could right-click on `>>` to see that it is identical to:
case Just 3 of
Just _ -> Just 4
Nothing -> Nothing
Which would allow them to easily see that this expression will always evaluate to "Just 4" (and hopefully simplify it). Enlightened,
they could then hit ctrl-z to revert the code to its original state.
Haskell's purity and laziness makes it one of the only mainstream languages where equational reasoning is really possible, so this feature would play on Haskell's strengths and provide a feature that couldn't be replicated in other languages' IDEs. It would also allow new users to more easily dive in and understand certain abstractions (although how it would work in many cases would be hard to determine).