[erlang-questions] Pattern-matching function?

[Fredrik Linder]

Hi Richard

Could you give a code example of the second alternativ please? This question
is not related to first class pattern matching, I use alternative 1 to
generate mocks but would rather use alternative 2 as my mocks does not need
the preprocessor.

Thank you
/Fredrik

On Thu, Oct 30, 2008 at 9:30 AM, Richard Carlsson <richardc@REDACTED> wrote:

> Francois De Serres wrote:
> > Thank you Richard.
> > Any way to overcome this with parsing/binding/eval magic, even cumbersome
> ?
>
> Depending on what you need, in terms of efficiency (is the code going to
> be a bottleneck, or just executed occasionally?), and if you really need
> to do metaprogramming (often a big *if*), there are several options:
>
>  - Output the code as plain text to a .erl-file and call the compiler.
>   (Dead simple, and quite flexible - can use macros and includes for
>    "template" stuff so you don't need to generate that as well.)
>
>  - Generate the code as text in memory, call the scanner and parser, and
>   pass the result to compile:forms/2. You can't use the preprocessor
>   this way (it only works on files), but you don't have to go to disk,
>   and you don't need to learn about the syntax tree representation.
>
>  - Generate the syntax trees directly and pass to the compiler. Fiddly,
>   unreadable and hard to maintain, but low-overhead and full control.
>
> But doing compilation on the fly might not be suitable if you are going
> to do it a million times (unless you can reuse the same module name, you
> will have to generate unique module names for each new compilation, and
> who will then be responsible for deleting unused modules?). If the code
> you are going to run is short and not a bottleneck, it could be better
> to use erl_eval, for example like this:
>
> make_match_fun(Pstr) -> element(2, erl_eval:expr(hd(element(2,
> erl_parse:parse_exprs(element(2,
> erl_scan:string(lists:flatten(io_lib:format("fun (~s=__P) ->
> {matched,__P}; (_) -> fail end.", [Pstr]))))))), erl_eval:new_bindings())).
>
> This will, via erl_eval, produce a fun that can be used like you wanted:
>
> F = make_match_fun("{X,Y}")
>
> F({1,2}) ==> {matched, {1,2}}
>
> F({}) ==> fail
>
>    /Richard
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