[erlang-questions] How to create a function using merl given a list of clauses

[Frans Schneider]

Thanks Svilen, works now!
As always, it is in the docs but it sometimes requires reading it 
several times.

Frans

On 02/14/2018 08:47 PM, Svilen Ivanov wrote:
> On Wed, 2018-02-14 at 13:31 +0100, Frans Schneider wrote:
>> Dear list,
>>
>>   From a set of user defined rules, I want to create functions of the
>> form:
>>
>> fun([V1, V2]) ->
>>       fun () when V1 < 1, is_atom(V2) -> foo();
>>           () when V2 =/= b -> bar();
>>           () -> false
>>       end
>> end.
>>
>> The number of arguments, clauses and guards differ. Guards always
>> are
>> ANDed or andalso should be used.
>>
>> To build the internal fun I do something like:
>>
>> ...
>> -include_lib("syntax_tools/include/merl.hrl").
>> ...
>> tst() ->
>>       %% This is the end result I want
>>       V1 = 10,
>>       V2 = a,
>>       Func = ?Q(["fun () when _@REDACTED@ < 1, is_atom(_@REDACTED@) -> foo();",
>>              "    () when _@REDACTED@ =/= b -> bar();",
>>              "    () -> false ",
>>              "end"]),
>>       %% io:format("Func ~p~n", [Func]),
>>       merl:print(Func),
>>
>>       %% This is the data produced by some function
>>       Guard1 = ?Q("_@REDACTED@ < 1"),
>>       Guard2 = ?Q("is_atom(_@REDACTED@)"),
>>       Guard3 = ?Q("_@REDACTED@ =/= b"),
>>       GSeq1 = [Guard1, Guard2],
>>       GSeq2 = [Guard3],
>>
>>       %% Now start assembling the fun
>>       Clause1 = ?Q("() when _@REDACTED -> foo()"),
>>       Clause2 = ?Q("() when _@REDACTED -> bar()"),
>>       Clauses = [Clause1, Clause2],
>>       merl:print(Clauses),
>>
>>       %% And this gives the error: syntax error before: 'fun'"
>>       Func1 = ?Q("fun _@REDACTED end"),
>>       merl:print(Func1).
>>
>> I just can't figure out a way to make the last step assembling the
>> fun work.
>>
>> Your suggestions are highly appreciated!
>>
>> Frans
>>
> 
> You should look at "lifting" in http://erlang.org/doc/man/merl.html ,
> something like this seems to work:
> ----------------------------------------------------
> -include_lib("syntax_tools/include/merl.hrl").
> 
> tst() ->
>       V1 = merl:var('V1'),
>       V2 = merl:var('V2'),
> 
>       Guard1 = ?Q("_@REDACTED < 1"),
>       Guard2 = ?Q("is_atom(_@REDACTED)"),
>       Guard3 = ?Q("_@REDACTED =/= b"),
>       GSeq1 = [Guard1, Guard2],
>       GSeq2 = [Guard3],
> 
>       Clause1 = ?Q("() when _@REDACTED -> foo()"),
>       Clause2 = ?Q("() when _@REDACTED -> bar()"),
>       Clauses = [Clause1, Clause2],
> 
>       Func1 = ?Q("fun() -> _@REDACTED@Clauses end"),
> 
>       Func2 = ?Q("fun([_@REDACTED,_@REDACTED]) -> _@REDACTED end"),
> 
>       merl:print(Func2).
> ------------------------------------------------------
> 
> Svilen
>