Erlang Central

Difference between revisions of "String Eval"

From ErlangCentral Wiki

(clean up the example.)
Line 5:Line 5:
 
== Solution ==
 
== 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:  
+
Use erl_scan:string/1 to convert the string into a list of tokens, then use erl_parse:parse_exprs/1 to generate the Erlang intermediate representation, then finally use erl_eval:exprs/2 to generate the final output:  
 
<code>
 
<code>
1> {ok,A,B} = erl_scan:string("A = 1 + 2.").
+
eval(S,Environ) ->
{ok,[{var,1,'A'},{'=',1},{integer,1,1},{'+',1},{integer,1,2},{dot,1}],1}
+
    {ok,Scanned,_} = erl_scan:string(S),
2> A.
+
    {ok,Parsed} = erl_parse:parse_exprs(Scanned),
[{var,1,'A'},{'=',1},{integer,1,1},{'+',1},{integer,1,2},{dot,1}]
+
    erl_eval:exprs(Parsed,Environ).
3> {C,D} = erl_parse:parse_exprs(A).
+
 
{ok,[{match,1,{var,1,'A'},{op,1,'+',{integer,1,1},{integer,1,2}}}]}
+
1> eval("A = 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}]}
 
{value,3,[{'A',3}]}
 
</code>
 
</code>

Revision as of 10:44, 25 September 2006

Problem

You want to evaluate Erlang code stored in a string.

Solution

Use erl_scan:string/1 to convert the string into a list of tokens, then use erl_parse:parse_exprs/1 to generate the Erlang intermediate representation, then finally use erl_eval:exprs/2 to generate the final output:

eval(S,Environ) ->
    {ok,Scanned,_} = erl_scan:string(S),
    {ok,Parsed} = erl_parse:parse_exprs(Scanned),
    erl_eval:exprs(Parsed,Environ).

1> eval("A = 1 + 2.").
{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.