Reconsider haskell-indent

[gracjan]

@jstolarek made me aware of functionality in haskell-indent that is not present in haskell-indentation.

I was under the impression that haskell-indentation is evolved and improved form of haskell-indent and itself was superseeded by hi2 (later renamed back to haskell-indentation).

It does not seem to be the case.

The plan:

• see what is different
• find out features in haskell-indent
• see if haskell-indent is hackable in better or similar way as haskell-indentation

[jstolarek]

In the last couple of days I realized that blaming all these indentation problems entirely on haskell-indentation might not be fair. All these problems began for me after updating Emacs 24.3.1 to 24.5.1. This switch broke behaviour of haskell-indent (#1013), which forced me to switch to haskell-indentation. But now I see that indentation behaviour has generally changed in Emacs after the update: it behaves differently than it used to in LaTeX mode and text mode as well. So it seems that latest version of Emacs has changed its behaviour when it comes to indentation.

[jstolarek]

After a few minutes of investigation I learned that since version 24.4 Emacs comes with electric-indent-mode enabled by default. Disabling it restores haskell-indent's behaviour to normal. But it doesn't seem to affect haskell-indentation in any way. For now I am happy that I can once again use haskell-indent.

[gracjan]

I'm checking out haskell-indent code and this is my personal opinion so far. haskell-indent seems to be very cleanly written. It tries to find definitions and then separate definition parts so that each part may be indented according to its own indentation rules. There is a decision table and a bit of special logic here and there. Variable names are short and this does not help but then comments are to the point and clarify concepts.

There is no test suite, but it can be created.

Due to programming model with special cases for splits, decision table and other specail constructs I do not see immediately how to extend haskell-indent to support constructs in language that were created after haskell-indent was finished. Compare that haskell-indentation is a recursive descent parser that allows rather easy and schematic improvements.

There is a charm to the code so I think I'll push forward with test suite and then we will see where to take it from there.

[gracjan]

haskell-indent will suggestion function after this code:

function = Record
           { field = 123 }

but not after this:

function = Record
      { field = 123 }

haskell-indent will suggest z in line after this code:

fact n = do
  z <- let x = 5
       in return (x + 4)

Looks like there are plenty of bugs here and there.

[jstolarek]

Looks like there are plenty of bugs here and there.

Good point. But TBH these bugs never bothered me too much - pressing Tab cycles to another indent alternative.

[gracjan]

I guess you just learned to not see it just like other people have learned to not see issues with haskell-indentation.

[gracjan]

I added a test suite for haskell-indent. It does not look good but it is a good start if somebody would like to take over from there.

[gracjan]

Merging with #1065.