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Trauma
Asked: 2020-10-10 19:54:02 +0800 CST 2020-10-10 19:54:02 +0800 CST

Nested generators: execution terminates earlier than expected

  • 772

I'm doing a test with generators, to split a string into tokens . But the result obtained is not as expected:

function *stringIterator( str ) {
    for( const ch of str ) {
        yield ch
    }
}

function parseString( iterator ) {
    var value = '';

    for( const ch of iterator ) {
        if( ch == "'" ) { break; }

        value += ch;
    }

    return { value };
}

function *tokenGenerator( input ) {
    iterator = stringIterator( input );

    for( const ch of iterator ) {
        switch( ch ) {
        case "'":
            yield parseString( iterator );
            break;
        }
    }
}

for( const token of tokenGenerator( "'hola, mundo''hadios'" ) ) {
    console.log( token );
}

The value obtained by console is

{ value: 'hello, world' }

However, what I hope to get is

{ value: 'hello, world' }
{ value: 'goodbye' }

There are 3 functions involved, 2 of them being generators and the third being a normal function :

  1. function *stringIterator( )

    Generator that loops through a string, returning one character at a time.

  2. function parseString( )

    Normal function that, using the first, has to accumulate characters in a string, and return the accumulated string when the character found is a '(single quote).

  3. function *tokenGenerator( )

    Generator function that creates a generator ( 1) from a string and should repeatedly call 2.

From the results obtained, it seems that as much 1as they 2work correctly, they fail 3(or my way of calling it).

  • What am I doing wrong ?

  • How do I solve it ?

javascript

1 Answers

  1. Best Answer

    The problem is that inside parseString()you are iterating again with for(...)and "breaking" the generator loop.

    Within a while loop , you must continue iterating with .next()to preserve the functionality of the generator, knowing that the function returns a result with two properties: value, for each value in iteration and done, true when it ends or false when there are still pending values.

    Reference: Yield - Description

    function *stringIterator( str ) {
    for( const ch of str ) {
        yield ch
    }
}

function parseString( iterator ) {
    var value = '';
    while(true) {
        // Obtener resultado para analizar
        let resp = iterator.next();
        // Terminó la iteración o es caracter de "fin"?
        if(resp.done || resp.value == "'") {
            // Salir del ciclo
            break;
        }
        // Agregar caracter actual a cadena
        value += resp.value;
    }
    return { value };
}

function *tokenGenerator( input ) {
    iterator = stringIterator( input );

    for( const ch of iterator ) {
        switch( ch ) {
        case "'":
            yield parseString( iterator );
            break;
        }
    }
}

for( const token of tokenGenerator( "'hola, mundo''hadios'" ) ) {
    console.log( token );
}

    • 3