CamlTrax
This page is about CamlTrax, a layout preprocessor for OCaml.

OCaml is a fantastic programming language, but I, like many Haskellers, wish it had layout-based parsing to avoid all the parens. A friend recently introduced me to OCaml+TWT, a layout preprocessor for OCaml, but I don't like how it handles layout, and I have trouble, e.g., reading nested "if"s. So, I wrote CamlTrax, my own preprocessor. It is at least as retarded as OCaml+TWT, but in a way that I find less irritating.

• 2008-09-26–Another little bug fix. ("[" and "{" behaved strangely, e.g., "x.[y]" would be rewritten to "[x.y]"; this was because I neglected to emit the previous token when I encountered these braces.)
• 2008-09-01–I've uploaded a minor bug fix. (There was a problem with the position adjustment used for "match" expressions; it allowed the cases to be directly beneath the "match" keyword, but the expression following the ";" was incorrectly grouped with the last case; this has been fixed.)
• 2008-08-17–I've been working on some modifications to CamlTrax. Primarily, these are aimed at cleaning up the code and at allowing more flexible use of braces (i.e., "{", "}", "[", and "]" are actually recognised as braces that must be nested properly). Keywords need to be escaped differently, so this is not backwards compatible, but there is a flag to engage compatibility with the previous syntax.
• 2008-04-05–Uploaded the latest version of CamlTrax; it incorporates some minor fixes; I've been using this version for several months and consider it (relatively) stable.
• 2007-07-18–Totally rewrote CamlTrax; it's much less retarded now, and the changes are totally not backwards compatible.
• 2007-07-07–Uploaded first release of CamlTrax.

• CamlTrax is open-source; the only available download is the OCaml source (requires OCaml+TWT (or CamlTrax!) to compile):
  • source (tgz)

Demonstrations

Part of the source code for CamlTrax. while (snd $ Stack.top st) <> lparenF do emit $ rparen $ Stack.top st ; ignore $ Stack.pop st ; if (snd $ Stack.top st) = lparenF then emit $ rparen $ Stack.top st ; ignore $ Stack.pop st ; advance t

Generated OCaml equivalent. (* Generated by * * CamlTrax * * Version 3 *) ( while (snd ( Stack.top st )) <> lparenF do emit ( rparen ( Stack.top st ; ) ) ignore ( Stack.pop st ) done ); ( if (snd ( Stack.top st )) = lparenF then ( emit ( rparen ( Stack.top st ) ); ignore ( Stack.pop st ) ) ); advance t

Composition & infix application. fun env -> map $ second $ drop . eval env . lift

Generated OCaml equivalent. (* Generated by * * CamlTrax * * Version 3 *) fun env -> map ( second ( drop ++ eval env ++ lift ) )

Nested "if"s. if is_tcon c then KType else if is_pcon c then KProp else raise $ TypeError "Kind error."

Generated OCaml equivalent. (* Generated by * * CamlTrax * * Version 3 *) ( if is_tcon c then ( KType ) else ( ( if is_pcon c then ( KProp ) else ( raise ( TypeError "Kind error." ) ) ) ) )

More Information

If you decide to try out CamlTrax, there are a few things you should keep in mind.

• You can disable layout for a particular keyword by enclosing it in "!"'s. There mustn't be spaces between the braces and the keyword. This is obligatory for the top-level "let"'s occurring within a module definition.
• CamlTrax is column-based and operates by looking for "pushers"—keywords whose scopes will be explicitly delimited with parens in the generated code—and putting them on the stack when they're found. Any subsequent line that has non-space text at or before the column where the pusher was found closes the block (i.e., triggers right-paren insertion). This includes several normal OCaml keywords and a couple "pseudo-keywords" (e.g., "$")
• Scope can be explicitly delimited with parens, but these do not disable layout; rather, they force close-scope parens for all pushers encountered between them (assuming those pushers have not already been closed).
• "." is a huge hack. I always define function composition as "++" in OCaml, and "." (occurring surrounded by whitespace) is replaced by "++"; if you've not defined this binding, don't use ".".

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