[erlang-questions] solved: Pattern-matching function?

Hynek Vychodil <>
Thu Oct 30 18:41:34 CET 2008


On Thu, Oct 30, 2008 at 5:25 PM, Francois De Serres <>wrote:

> I'll investigate further both of these options:
>
> A) Match Specifications with ets:match_spec_run/2 (thanks to Michael
> Radford):
> slightly awkward to use (match spec syntax), but straightforward to
> implement.
> <http://www.erlang.org/doc/apps/erts/match_spec.html>
>
> B) ad-hoc matching functions generated with erl_eval (thanks to Richard
> Carlsson):
> metaprogramming magic, but the resulting API is lean.
> 8<--------
> 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())).
> F = make_match_fun("{X,Y}")
> F({1,2}) ==> {matched, {1,2}}
> F({}) ==> fail
> 8<--------
>
> Thanks to all for your precious help,
> --
> François
>
>
One of most important thing in Erlang is reliability and readability which
goes in Erlang together. Compare this code and yours:

make_match_fun(Pstr) ->
    FStr = lists:flatten(io_lib:format(
        "fun (~s=__P) -> {matched,__P}; (_) -> fail end.", [Pstr]
      )),
    {ok, Tokens, _Line} = erl_scan:string(FStr),
    {ok, [Expr]} = erl_parse:parse_exprs(Tokens),
    {value, Fun, _} = erl_eval:expr(Expr, erl_eval:new_bindings()),
    Fun.

Difference is not only you can see what is going on, but if any fail, you
get valuable error message. It fails fast and as result code is more
reliable.


> > Hi there.
> >
> > I need a function that actually does pattern matching: pm(Tuple,
> > Pattern)->MatchedPattern|error
> > > pm({a,b,c}, {a,X,Y}).
> > {a,b,c}
> > >X.
> > b
> > > pm({a,b,c}, {b,X,Y}).
> > {error, badmatch}
> > > pm({a,b,c}, {a,X}).
> > {error, badmatch}
> > etc.
> >
> > This trivial code works only with the empty tuple/pattern :
> > pm(T,P) ->
> >     P = T, P.
> > because pattern variables (X, Y) are unbound at function call.
> >
> > I tried to find a hint in the libs with no avail.
> > If anyone has a lead, can you please put me on track?
> >
> > Thanks,
> > --
> > Francois
> >
> > --
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-- 
--Hynek (Pichi) Vychodil
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