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autophile
Posted: Mon Dec 03, 2007 1:59 am Reply with quote
User Joined: 12 Jan 2007 Posts: 31 Location: Maryland, USA
Hi all,

Is there any way to do something like this:

Code:

% Input = string()
% Prefix = string()

Prefix = generate_complicated_prefix(),
case Input of
  [ Prefix | Tail ] ->
    do_something(Tail);
  [ $X, Prefix | Tail ] ->
    do_something_else(Tail)
end.


Obviously this won't work, because the case clauses turn out to be deep lists -- I need a flattened list to match against. Any ideas as to the Erlangy way to do this? I'm kind of dreading that the answer will turn out to require lists module calls Sad

Thanks,

--Rob
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michal
Posted: Mon Dec 03, 2007 12:39 pm Reply with quote
User Joined: 20 Jul 2006 Posts: 44 Location: London
If all your lists are short, you can rewrite your case statement as:

Code:

case Input of
  Prefix ++ Tail ->
    do_something(Tail);
  [$X|Prefix] ++ Tail ->
    do_something_else(Tail)
end.


However ++ operator is quite expensive and if Tail or Prefix are long lists it will impact the performance.

Michal

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autophile
Posted: Mon Dec 03, 2007 1:55 pm Reply with quote
User Joined: 12 Jan 2007 Posts: 31 Location: Maryland, USA
According to the erl shell, Prefix ++ Tail is an illegal pattern. I guess all variables in a ++ expression have to be bound, and Tail is not bound.

Any other ideas?
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Adam
Posted: Tue Dec 11, 2007 2:34 pm Reply with quote
User Joined: 23 Aug 2006 Posts: 71 Location: London
In a function header you can write:

Code:
func("test" ++ Rest) ->
    Rest.


Which in the case of the input "teststring" will bind Rest to "string". Should be the same for any pattern matching.

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autophile
Posted: Tue Dec 11, 2007 8:32 pm Reply with quote
User Joined: 12 Jan 2007 Posts: 31 Location: Maryland, USA
Well, I'm not matching against a fixed string. I'm matching against a variable.
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bluefly
Posted: Mon Jan 07, 2008 12:08 am Reply with quote
User Joined: 06 Jan 2008 Posts: 10
I am an Erlang newbie, but I feel I have a fair handle on the language in general. Here would be my approach to your problem. I am assuming that you want to match a sublist in a list (or a substring in a string).

There is no way to match a sublist in a list in a guard expression. For one, there is no syntax for it (the syntax is limited to things like the head vs tail of the list). For another, there is no BIF (akin to is_list(), is_integer()) or operator that does this; this means you cannot do this matching in a guard expression either. These two issues knock out unique function header matching and use of "when".

Remember that guards are not allowed to have potential side effects, so they are extra limited.

However, we can still store the results of a match in a variable and then make simple comparisons against the variable.

Here is some tested code that demonstrates the concept, including it crashing when we reach an unexpected prefix situation:

Code:
-module(junk).
-export([start/0]).

start() ->
    Prefix = generate_complicated_prefix(),

    Input = Prefix ++ "this is the tail",
    io:format("Input with prefix: ~s~n", [Input]),
    process_input(Prefix, Input),

    XInput = [$X | Input],
    io:format("Input with complicated prefix: ~s~n", [XInput]),
    process_input(Prefix, XInput),

    BadInput = [$B | Input],
    io:format("Bad input: ~s~n", [BadInput]),
    process_input(Prefix, BadInput).

generate_complicated_prefix() ->
    "some complicated prefix 123_yeah".

process_input(Prefix, Input) ->
    {Prefix_Type, Tail} = get_tail(Prefix, Input),
    case Prefix_Type of
        simple -> do_something(Tail);
        complicated -> do_something_else(Tail)
    end.

get_tail(Prefix, Input) ->
    Is_Simple_Prefix = string:equal(
        string:substr(Input, 1, length(Prefix)),
        Prefix
    ),
    Is_Complicated_Prefix = string:equal(
        string:substr(Input, 1, length(Prefix) + 1),
        [$X | Prefix]
    ),

    if
        Is_Simple_Prefix -> get_tail(Prefix, Input, simple);
        Is_Complicated_Prefix -> get_tail(Prefix, Input, complicated)
    end.

get_tail(Prefix, Input, simple) ->
    {simple, string:substr(Input, length(Prefix) + 1)};

get_tail(Prefix, Input, complicated) ->
    {complicated, string:substr(Input, length(Prefix) + 2)}.

do_something(Tail) ->
    io:format("do_something(~s)~n", [Tail]).

do_something_else(Tail) ->
    io:format("do_something_else(~s)~n", [Tail]).


I tried to make this elegant, but I welcome comments on how to improve this.
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