Dynamic pattern matching

Question

How can I do dynamic pattern matching in Erlang?

Supose I have the function filter/2 :

filter(Pattern, Array)

where Pattern is a string with the pattern I want to match (e.g "{book, _ }" or "{ebook, _ }") typed by an user and Array is an array of heterogenous elements (e.g {dvd, "The Godfather" } , {book, "The Hitchhiker's Guide to the Galaxy" }, {dvd, "The Lord of Rings"}, etc) Then I would like filter/2 above to return the array of elements in Array that match Pattern.

I've tried some ideas with erl_eval without any sucess...

tks in advance.

Answer 1

You can use lists:filter/2 to do the filtering part. Converting the string to code is a different matter. Are all the patterns in the form of {atom, _}? If so, you might be able to store the atom and pass that into the closure argument of lists:filter.

Answer 2

With little bit documentation study:

Eval = fun(S) -> {ok, T, _} = erl_scan:string(S), {ok,[A]} = erl_parse:parse_exprs(T), {value, V, _} = erl_eval:expr(A,[]), V end,
FilterGen = fun(X) -> Eval(lists:flatten(["fun(",X,")->true;(_)->false end."])) end,
filter(FilterGen("{book, _}"), [{dvd, "The Godfather" } , {book, "The Hitchhiker's Guide to the Galaxy" }, {dvd, "The Lord of Rings"}]).
[{book,"The Hitchhiker's Guide to the Galaxy"}]

Answer 3

Several possibilities come to the mind, depending on how dynamic the patterns are and what features you need in your patterns:

1. If you need exactly the syntax of erlang patterns and the pattern doesnt't change very often. You could create the matching source code and write it to a file. Use compile:file to create a binary and load this with code:load_binary.

   • Advantage: Very fast matching

   • Disadvantage: overhead when pattern changes

2. Stuff the data from Array into ETS and use match specifications to get out the data

   • You might use fun2ms to help create the match specification. But fun2ms normally is used as a parse transfor during compile time. There is also a mode used by the shell that can be made to work from strings with the help of the parser probably. For details see ms_transform

3. There might also be some way to use qlc but I didn't look into this in detail.

In any case be careful to sanitize your matching data if it comes from untrusted sources!

Answer 4

Is there any special reason why you want the pattern in a string?

Patterns as such don't exist in Erlang, they can really only occur in code. An alternative is to use the same conventions as with ETS match and select and write your own match function. It is really quite simple. The ETS convention uses a term to represent a pattern where the atoms '$1', '$2', etc are used as variables which can be bound and tested, and '_' is the don't care variable. So your example patterns would become:

{book,'_'}
{ebook,'_'}
{dvd,"The Godfather"}

This is probably the most efficient way of doing it. There is the possibility of using match specifications here but it would complicate the code. It depends on how complicated matching you need.

EDIT: I add without comment code for part of the matcher:

%% match(Pattern, Value) -> {yes,Bindings} | no.

match(Pat, Val) ->
    match(Pat, Val, orddict:new()).

match([H|T], [V|Vs], Bs0) ->
    case match(H, V, Bs0) of
        {yes,Bs1} -> match(T, Vs, Bs1);
        no -> no
    end;
match('_', _, Bs) -> {yes,Bs};                  %Don't care variable
match(P, V, Bs) when is_atom(P) ->
    case is_variable(P) of
        true -> match_var(P, V, Bs);            %Variable atom like '$1'
        false ->
            %% P just an atom.
            if P =:= V -> {yes,Bs};
               true -> no
            end
    end.

match_var(P, V, Bs) ->
    case orddict:find(P, Bs) of
        {ok,B} when B =:= V -> {yes,Bs};
        {ok,_} -> no;
        error -> {yes,orddict:store(P, V, Bs)}
    end.