Transducers (in javascript) in action.

example-transducers-v3.js

// Getting some stats from https://dev.to/dashboard
// Inspired by this: https://dev.to/tylerlwsmith/get-your-dev-2020-year-in-review-scraping-data-using-the-console-3gen

function compose(...fns) {
  const apply = (arg, fn) => fn(arg);
  return (initial) => fns.reduceRight(apply, initial);
}

function to_transducer(reducer) {
  if(typeof reducer['@@transducer/step'] == 'function') {
    return reducer;
  }

  return {
    '@@transducer/init': function() {
      throw new Error('Method not implemented');
    },
    '@@transducer/result': function(state) {
      return state;
    },
    '@@transducer/step': function(state, value) {
      return reducer(state, value);
    }
  };
}

function reduce(transducer, initial, collection) {
  transducer = to_transducer(transducer);

  if(arguments.length === 2) {
    collection = initial;
    initial = transducer['@@transducer/init']();
  }

  let state = initial;

  for(let value of collection) {
    state = transducer['@@transducer/step'](state, value);

    if(state != null && state['@@transducer/reduced']) {
      state = state['@@transducer/value'];
      break;
    }
  }

  return transducer['@@transducer/result'](state);
}

function filter(predicate, next) {
  if(arguments.length === 1) {
    return (_next) => filter(predicate, _next);
  }

  return {
    '@@transducer/init': function() {
      return next['@@transducer/init']();
    },
    '@@transducer/result': function(state) {
      return next['@@transducer/result'](state);
    },
    '@@transducer/step': function(state, value) {
      if(predicate(value)) {
        return next['@@transducer/step'](state, value);
      }

      return state;
    },
  };
}

function map(transform, next) {
  if(arguments.length === 1) {
    return (_next) => map(transform, _next);
  }

  return {
    '@@transducer/init': function() {
      return next['@@transducer/init']();
    },
    '@@transducer/result': function(state) {
      return next['@@transducer/result'](state);
    },
    '@@transducer/step': function(state, value) {
      return next['@@transducer/step'](state, transform(value));
    },
  };
}

function transduce(combine, initial, transducer, collection) {
  return reduce(
    transducer(to_transducer(combine)),
    initial,
    collection
  );
}

var ensure_number = (value) => 
  Number.isNaN(value) ? 0 : value;

var get_text = (element) => (query) =>
  element.querySelector(query).innerText;

var get_data = function(story) {
  const text = get_text(story);
  return {
    published: new Date(story.querySelector('time').dateTime),
    reactions: Number(text('[title="Reactions"]')),
    comments: Number(text('[title="Comments"]')),
    views: Number(text('[title="Views"]')),
  };
}

var published_2020 = function(story) {
  return story.published.getFullYear() === 2020;
}

var add_stats = function(state, value) {
  return {
    reactions: state.reactions + value.reactions,
    comments: state.comments + value.comments,
    views: state.views + ensure_number(value.views)
  };
};

var transducer = compose(
  map(get_data),
  filter(published_2020)
);

var stats = {
  reactions: 0,
  comments: 0,
  views: 0,
};

var stories = document.querySelectorAll(
  '.dashboard-story:not(.story-unpublished)'
);

transduce(add_stats, stats, transducer, stories);