Convert a string into a fun

Question

I'm trying to get around a problem with file:consult/1 not allowing tuples with fun in them like in this example:

{add_one, fun(X) -> X+1 end}.

To get around this I'm considering writing the fun inside a string and evaluating it

{add_one, "fun(X) -> X+1 end"}.

The question is. How do I convert the string into a fun?

Answer 1

Maybe by using the erl_eval module?

Answer 2

parse_fun_expr(S) ->
  {ok, Ts, _} = erl_scan:string(S),
  {ok, Exprs} = erl_parse:parse_exprs(Ts),
  {value, Fun, _} = erl_eval:exprs(Exprs, []),
  Fun.

Note that you need a period at the end of your fun expression, e.g. S = "fun(X) -> X + 1 end.".

Answer 3

I'd like to point out that Zed's answer creates an interpreted fun. When the fun is called it enters the evaluator which starts to evaluates the abstract syntax tree returned by erl_parse:parse_exprs/1 that it has captured. Looking at the fun created:

11> erlang:fun_info(Fun, env).
{env,[[],none,none,
      [{clause,1,
               [{var,1,'X'}],
               [],
               [{op,1,'+',{var,1,'X'},{integer,1,1}}]}]]}
12> erlang:fun_info(Fun, module).
{module,erl_eval}

One can see that it has closed over the parsed abstract syntax tree as seen in the env info, and it is a fun created inside erlang_eval as seen in the module info.

It is possible to use the erlang compiler to create a compiled module at runtime, and a pointer toward that is compile:forms/2 and code:load_binary/3. But the details of that should probably go into another stackoverflow question.

Answer 4

file:script/1 almost does what you want - it evaluates a series of erlang expressions from a file and returns the last result. You could use it in place of file:consult/1 but you'd need to change the format of the file from "term. term. term." giving [term, term ,term] to "[term, term , term]." giving [term, term, term] - place a single expression in the file instead of a sequence.