On Tue, 26 Aug 2003 11:58:20 -0700 John Meacham wrote:

On Tue, Aug 26, 2003 at 02:00:32PM +0200, Per Larsson wrote:

...

I have problems finding a pleasing indentation style for haskell code. Especially nested do-blocks have a tendency to run away to the right margin. When looking on source code from experienced haskell programmers, there seems not to be any consensus at all, everyone uses their own convention and in many cases one changes style in the same module.

Also, the automatic tools are problematic: the emacs mode I'm using bails out in certain contexts and there are few user customizations available. The haskell-src module in the GHC library offers a parser and pretty-printer for haskell code with nice options for customizing the indentation, but it can't handle comments which is a problem if you want to use it as a basis for implementing a indentation tool.

Is there anyone who have given this some thought and have some suggestions for a consistent indentation style and/or desktop tools which I'm not aware of?

I highly recommend the always-enter model. which means when you are using layout you always do a linebreak after any block forming construct(do, let, while, ...) and indent one more softtab level than the surrounding code. this has a number of advantages: * no need for special modes or editor support * indents are always an integral number of softtabs. * code doesn't run off the right side of the screen since your indentation level is relative to the start of the line, not the expression that started it. * cut-n-paste of code blocks is easier.

examples of what I mean can be seen here: http://repetae.net/john/computer/haskell/

I have known several people to get turned off of haskell when trying to recreate the indent style usually found in publications by hand...

John

That style is pretty much what I use. Looking at the "examples" (well, one, Format.hs) though, there are a few things I do differently and typically see done differently. The first is that I always put the where for local function definitions on a new line, e.g. foo x = x + a + b where a = 1 + x b = 2 * x I find this a lot clearer and prettier than, foo x = x + a + b where a = 1 + x b = 2 * x The only awkward thing I can think of is rearranging the local definitions; the first case needs special handling. However, I don't typically do that. Also to be clear, I still write, class Foo a where foo :: a -> Bool Another thing I commonly see and use for data types with many alternates is the following style, data AST = Const Int | Var String | Lam String AST | App AST AST | Let String AST AST | Add AST AST Some other tricky instances are large pattern matches, in which case I put the = on the next line indented, e.g. foo (Let v1 e1 (Let v2 e2 (Lam x body))) = ... With my particular style if you want where blocks after a do block then the indentation won't be right if you go to a tabstop. Personally, I don't care about exactly lining my code up on tabstops, so I use two spaces (four spaces being a tab for me) in those cases, e.g. foo = do a <- bar b <- baz return (a+b) where bar = return 1 baz = return 2 Another difficult case that I haven't solidified a style for is large type signatures. Finally, you can use higher-order functions to handle some cases pretty cleanly. For example, I use 'either' quite a bit.