Brian Hulley wrote:
Here is my proposed layout rule:
1) All layout keywords (where, of, let, do) must either be followed by a single element of the corresponding block type, and explicit block introduced by '{', or a layout block whose first line starts on the *next* line
I wouldn't have much trouble adapting to that.
and whose indentation is accomplished *only* by tabs
You can't be serious. This would cause far more problems than the current rule.
I would also make it that explicit braces are not allowed to switch off the layout rule (ie they can be used within a layout),
I don't understand. What does "used within a layout" mean?
multiline strings would not be permitted,
They aren't now, except with \ escapes. A stray " will be caught on the same line unless the line happens to end with \ and the next line happens to begin with \, which is exceedingly unusual.
and multiline comments would not be permitted (pragmas could easily be used just by using --#)
But --# doesn't introduce a comment. And this would make UNPACK pragmas rather inconvenient to use.
1) When you see a ';' you could immediately tell which block it belongs to by looking backwards till the next '{'
I guess that might be helpful, but it doesn't seem easier than looking left to the beginning of the current line and then up to the first less-indented line.
2) Variable width fonts can be used,
They can be used now, if you adhere to a certain style, but not everyone likes that style. I wrote in C++ with a variable width font and tabs at one time, but eventually went back to fixed width. One reason was that I couldn't use comment layout conventions that tend (in my experience) to improve readability more than monospacing hurts it. Another reason was that glyph widths appropriate to natural languages didn't work all that well for source code. Spaces are much more important in source code than in natural language, for example. A proportional font designed for source code would be nice, but I haven't found one yet. Stroustrup used a mixture of proportional and monospaced glyphs in _The C++ Programming Language_ and it worked well.
or different font faces to represent different sorts of identifier eg class names, tycons, value constructors, operators like `seq` as opposed to seq etc
Lots of editors do this with monospaced fonts; I think it's orthogonal to the layout issue.
3) Using only tabs ensures that vertical alignment goes to the same position on the screen regardless of the font and tabs could even have different widths just like in a wordprocessor
Requiring tabs is a really bad idea. Just forget it. Seriously.
4) Any keypress has a localised effect on the parse tree of the buffer as a whole ( { " no longer kill everything which follows and there would be no {- )
I don't understand why this is an advantage. If you have an editor that highlights comments in green, then large sections of the program will flash green while you type a {- -} comment, which might be annoying, but it also means you'll never forget to close the comment, so the practical benefit of forbidding {- -}, as opposed to simply not typing it yourself, seems nil.
5) It paves the way for a much more immersive editing environment, but I can't say more about this at the moment because I haven't finished writing it yet and it will be a commercial product :-)))
I guess everything has been leading up to this, but my reaction is that it renders the whole debate irrelevant. The only reason layout exists in the first place is to make source code look good in ordinary text editors. If you have a high-level source code editor that manipulates the AST, then you don't need layout, or tabs, or any of that silly ASCII stuff. The only time you need to worry about layout is when interoperating with implementations that use the concrete syntax, and then there's nothing to stop you from exporting in any style you like. And when importing, there's no reason to place restrictions on Haskell's layout rule, because the visual layout you display in the editor need have no connection to the layout of the imported file.
Using my self-imposed layout rule I'm currently editing all my Haskell code in a standard text editor using tabs set to 4 spaces and a variable width font and have no problems.
Which is the best argument for keeping the current rule! If it were changed as you propose, then someday Hugh Briley would come along and complain that Haskell's layout syntax squandered screen space---but he *wouldn't* be able to program in his preferred style, because it would no longer be allowed. Religious freedom is a good thing. {- Ben -}