the Strathclyde Haskell Enhancement

I wrote she over a couple of weekends in my flat overlooking Kelvingrove Park and Glasgow Uni. Brute force and ignorance is the philosophy at work. The hardest part was getting the layout rule to behave sensibly. If you want to have a crack at this sort of thing, take a look at HaLay.hs for a looyah experience — far too big and not clever enough, but it's some sort of approximation.

It would have been too much work to write a full Haskell parser. I could have tried tweaking Niklas Broberg's toolkit but my appetite for negotiating with happy is limited. I used a bargain basement parser combinator library Parsley.lhs which took no time flat to write, but probably longer to run. I'm sure a much quicker drop-in replacement is out there somewhere. Note, however, that my machinery is very careful to retain the whole of the layout information, so that she can reconstitute the file with the minimum of disturbance. Hat-tip to the HaRe crew.

The low-budget alternative to parsing is markup, and that's what I do: I group the tokens according to the bracket/layout structure, with layout regions chopped up into their logical lines (interspersed with hunks of whitespace/comment). I also crudely chunk out the regions which live in the type syntax, and in the kind syntax, as I need to treat them specially! Note that parser combinators work perfectly well on nested syntaxes. I also wrote a rock-bottom rewriting strategy combinator to do the heavy lifting. As a result, TypesToKinds is super-cheap.

The type-level version of data is made by declaring dummy type constructors the same arity as your data constructors: she prepends SheTy (sorry, Shilpa) to the names of your prefix constructors and :$#$#$# (expletive deleted) to your infix constructors. By the way, you now need {-# LANGUAGE TypeOperators #-} if you want to declare infix data constructors!

Pattern synonyms are a bit of a hack. They only come in prefix: you won't catch me implementing operator precedence in a hurry. The upshot is that we only need to look for Big Letters in non-type regions and we're basically there: then it's on-the-fly application parsing. There's no checking of linearity (or anything!) but your pattern synonym is left in the file as a function with patsy prepended to its name, like Lee Harvey Oswald.