The eval statement

Overview

Certain J services provide control functions on the J interpreter. The "eval" service is a powerful feature of an interpreted language.

Other services exist, which can help debug a J program, stop the execution of a J program or obtain information on the current execution context.

eval

The "eval" service is used to calculate a character String dynamically, by considering its contents as being composed of J instructions.

When the String is evaluated, the current object does not change, it is the one in use when the eval is called.

Note 1: Declarations of local variables are authorized in the String to be evaluated.

Note 2: A return statement in the evaluated String provokes a return from the method (non anonymous) that contains the eval statement.

Example of eval

We are going to use "eval" to read the variable values previously stored in a file (Storage example):

Object:Storage();

{

outStream Fic;

Fic.open ("Example");

Fic.write ("i1 = 15; s1 = "hello";

b1=false");

Fic.close();

Recover();

} – End of Storage

Object:Recover();

{

String read _buffer;

inStream Fic;

int i1;

String s1;

boolean b1;

Fic.open ("Example");

Fic.read (read_buffer);

eval (read_buffer);

//at this stage, the variables i1, s1, b1 are assigned.

StdOut.write ("i1=",i1,NL,

"s1=",s1,NL

"b1=",b1,NL);

}