[erlang-questions] Backtracking in Erlang, part 1 - control

Hynek Vychodil <>
Wed Mar 18 09:19:09 CET 2009

Sorry, I did not read carefully your post. It is not question, it is answer

On Wed, Mar 18, 2009 at 9:16 AM, Hynek Vychodil <>wrote:

> Have you seen 2.6.3 Parsing
> http://www.erlang.org/doc/programming_examples/funs.html#2.6?
> I think higher order function elegantly solve this even it can be slower
> than straight forward approach.
> 2009/3/18 Robert Virding <>
>> Sometimes you need to be able to do search algorithms which can step back
>> and try other alternate ways of finding a solution, "backtracking".
>> Generally this is difficult to do in functional languages like Erlang (most
>> other languages too for that matter). A classic example of this type of
>> problem is parsing ambiguous grammars.
>> Prolog syntax is like that, in some cases you need almost indefinite
>> look-ahead before you can decide how input is to be parsed. When I
>> implemented Erlog (a prolog in and for Erlang) I decided to do the parser in
>> Erlang so that it could run stand-alone and so I ran straight into this
>> problem.
>> Here is *a* solution to the problem. It is in two parts, the first
>> describing the control structures and the second (more difficult) describing
>> handling of data. There may also be a third part discussing some problems
>> with the solution to control, if anyone is still interested.
>> The main problem we have to overcome is that normally when you choose a
>> path you "commit" to it and can't go back and try an alternate path. For
>> example in:
>>     case get_a_value() of
>>         Choice1 -> ... ;
>>         Choice2 -> ... ;
>>         ...
>>     end
>> once you have chosen a path, say Choice1, there is no (simple) way to go
>> back try Choice2 if you later decide that Choice1 was wrong. We can't
>> backtrack.
>> Another problem is that if we have:
>>     a() -> foo(), bar(), baz().
>> and once we have completed foo() then even if it gave an answer which
>> later was shown to be wrong there is no way to step back into it, backtrack,
>> and let it give a new answer.
>> In this first section we will (almost) completely ignore arguments and
>> return values.
>> When looking at control we will (almost) completely ignore arguments and
>> returning values. We will use the convention that functions return either
>> '{succeed,Value}' when they succeed or 'fail' when they can't find a
>> solution.
>> The trick is to ensure that functions only return at all when they fail,
>> otherwise they just continue by explicitly calling the next thing which
>> should be done. We will do this by using a Continue Passing Style (CPS)
>> where each function will have an argument which is the next function call to
>> continue. (CPS is not new and is generally known) So functions will usually
>> look like this:
>>     a(Next) ->
>>         <do our stuff>,
>>         OurNext = fun () -> even_more_stuff(Next) end,
>>         more_stuff(OurNext).
>> We will use the convention that when a function fails and cannot go on it
>> will return 'fail'. When *the* solution has been found then the final test
>> will return {succeed,Value}. So the top level function will look something
>> like:
>> top_call() ->
>>     Succeed = fun () -> {succeed,yes} end,        %We have succeeded
>>     find_solution(Succeed).
>> So now our program will keep going forwards until it finally calls the
>> Succeed fun when it all returns back to the initial caller.
>> The next problem is to introduce 'choice points' where we can choose one
>> path but still allow the code to backtrack and make another choice. This is
>> the done in the function cp/1 where the argument is a list functions which
>> are separate paths to choose from:
>>     cp([fun () -> do_stuff_a(Next) end,
>>         fun () -> do_stuff_b(Next) end,
>>         fun () -> do_stuff_c(Next) end,
>>         ...]).
>> Cp/1 will try the functions in order, if one succeeds then the choice
>> point will succeed and return the same succeed value, if it fails then the
>> next function in the list will be tried, until no more exist when the choice
>> point will fail: The code for cp is simple:
>>     cp([G|Gs]) ->
>>         case G() of
>>             {succeed,Value} -> {succeed,Value};
>>             fail -> cp(Gs)
>>         end;
>>     cp([]) -> fail.
>> That's it, we're done. A lot of explanation for very little code.
>> A slight variation is to not chain the continuation together as shown here
>> but to have Next as a list of functions to call. Instead of directly calling
>> Next you call next/1 with argument Next, next(Next), where next/1 is defined
>> as:
>>     next([N|Ns]) -> N(Ns).
>> There is no functional difference it just make writing some code a bit
>> easier. For example the function defined above:
>>     a() -> foo(), bar(), baz().
>> would look like this in the normal way:
>>     a(Next) ->
>>         Next1 = fun () -> baz(Next) end,
>>         Next2 = fun () -> bar(Next1) end,
>>         foo(Next2).
>> while using the list variant it would look like:
>>     a(Next) ->
>>         foo([fun (Ns) -> bar(Ns) end,
>>              fun (Ns) -> baz(Ns) end | Next]).
>> which can be easier to read, especially if there many calls in the
>> continuation.
>> After I introduce handling data in the next installment I can give a
>> proper example.
>> I have also posted this to my blogg for those who prefer to read there,
>> Robert
>> _______________________________________________
>> erlang-questions mailing list
>> http://www.erlang.org/mailman/listinfo/erlang-questions
> --
> --Hynek (Pichi) Vychodil
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--Hynek (Pichi) Vychodil

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