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11.01.linesgenerator.js
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11.01.linesgenerator.js
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// A generator to yield the lines of the string s one at a time.
// Note that we don't use s.split(), because that would process the entire
// string at once, allocating an array, and we want to be lazy instead.
function eachline(s) {
let p;
while((p = s.indexOf('\n')) != -1) {
yield s.substring(0,p);
s = s.substring(p+1);
}
if (s.length > 0) yield s;
}
// A generator function that yields f(x) for each element x of the iterable i
function map(i, f) {
for(let x in i) yield f(x);
}
// A generator function that yields the elements of i for which f(x) is true
function select(i, f) {
for(let x in i) {
if (f(x)) yield x;
}
}
// Start with a string of text to process
let text = " #comment \n \n hello \nworld\n quit \n unreached \n";
// Now build up a pipeline of generators to process it.
// First, break the text into lines
let lines = eachline(text);
// Next, trim whitespace from the start and end of each line
let trimmed = map(lines, function(line) { return line.trim(); });
// Finally, ignore blank lines and comments
let nonblank = select(trimmed, function(line) {
return line.length > 0 && line[0] != "#"
});
// Now pull trimmed and filtered lines from the pipeline and process them,
// stopping when we see the line "quit".
for (let line in nonblank) {
if (line === "quit") break;
console.log(line);
}