[Haskell-cafe] A restricted subset of CPP included in a revision
of Haskell 98
lennart at augustsson.net
Thu Aug 17 20:44:19 EDT 2006
Even though I'm largely responsible for making CPP available in a
Haskell compiler I think it's an abomination. It should be avoided.
If we standardize it, people will use it even more. I think we
should discourage it instead, then looking at exactly what it's used
for and supplying sane versions of it.
On Aug 17, 2006, at 12:44 , Brian Smith wrote:
> I find it strange that right now almost every Haskell program
> directly or indirectly (through FPTOOLS) depends on CPP, yet there
> is no effort to replace CPP with something better or standardize
> its usage in Haskell. According to the following document, and my
> own limited experience in reading Haskell code, CPP is the most
> frequently used extension:
> I think that if we accepted that CPP was part of the language, we
> could then place some restrictions on its use to facilitate easier
> parsing. Here are some suggestions, off the top of my head:
> * #define can only be used for parameterless definitions
> * #define'd symbols are only visible to the preprocessor
> * #define can only give a symbol a value that is a valid
> preprocessor expression
> * #define can only appear above the module declaration
> * a preprocessor symbol, once defined, cannot be undefined or
> * #include and #undef are prohibited
> * The preprocessor can only be used at the top level. In
> particular, a prepropcessor conditional, #error, #warn, #line would
> not be allowed within the export list or within a top-level binding.
> * A Haskell program must assume that any top-level symbol
> definitions are constant over the entire program. For example, a
> program must not depend on having one module compiled with one set
> of command-line preprocessor symbol bindings and another module
> defined with a different set of bindings.
> * preprocessor directives must obey Haskell's layout rules. For
> example, an #if cannot be indented more than the bindings it contains.
> The result would be:
> * Syntax can be fully checked without knowing the values of any
> preprocessor symbols.
> * Preprocessor syntax can be added easily to a Haskell parser's BNF
> description of Haskell.
> * No tool will need to support per-file/module preprocessor symbol
> Again, all this is just off the top of my head. I am curious about
> what problems these restrictions might cause, especially for
> existing programs. I know that GHC itself uses some features that
> would be prohibited here. But, GHC is really difficult for tools to
> handle even with these restrictions on its source code. For now, I
> am more interested in the libraries in FPTOOLS and users' programs.
> What libraries/programs cannot easily be reorganizated to meet
> these restrictions? I suspect "#define'd symbols are only visible
> to the preprocessor" would be the most troublesome one.
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
More information about the Haskell-Cafe