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Evaluating Erlang Code in a String

Problem

You want to evaluate Erlang code stored in a string.

Solution

Use erl_scan:string to convert the string into a list of tokens, then use erl_parse:parse_exprs to generate the Erlang intermediate representation, then finally use erl_eval:exprs to generate the final output:

1> {ok,A,B} = erl_scan:string("A = 1 + 2.").
{ok,[{var,1,'A'},{'=',1},{integer,1,1},{'+',1},{integer,1,2},{dot,1}],1}
2> A.
[{var,1,'A'},{'=',1},{integer,1,1},{'+',1},{integer,1,2},{dot,1}]
3> {C,D} = erl_parse:parse_exprs(A).
{ok,[{match,1,{var,1,'A'},{op,1,'+',{integer,1,1},{integer,1,2}}}]}
4> D.
[{match,1,{var,1,'A'},{op,1,'+',{integer,1,1},{integer,1,2}}}]
5> erl_eval:exprs(D,[]).
{value,3,[{'A',3}]}

Now, this is an admittedly baroque way to determine the value of 1 + 2, but it does give you interesting access to the inner workings of the Erlang interpreter.

Discussion

(String input ports and read)

See Also

StringInterpolation

Contributors

-- BrentAFulgham - 21 Aug 2004