A better future for comprehensions

My presentation to TC39 on deferring comprehensions to post-ES6 in order to simplify and generalize the feature, based on recent input from Jafar Husain.

Transcript

1. A better future for comprehensions Dave Herman
2. [for  (x  of  y)  if  (p(x))  f(x)]   (for  (x  of  y)  if  (p(x))  f(x))
3. • Parallel JS is moving in the direction of parallel pipelines – a natural fit for comprehensions. • Three strikes and you refactor! • LINQ: one comprehension syntax, unbounded number of (user-definable) traversable datatypes.
4. let  a  =  for  (x  of  a1)                      for  (y  of  a2)                          if  (y  >  x)                              {  x,  y  };
5. let  i  =  for  (x  of  map1.keys())                      for  (y  of  map2.keys())                          if  (y  >  x)                              {  x,  y  };
6. let  p  =  for  (x  of  a1.parallel())                      for  (y  of  a2.parallel())                          if  (y  >  x)                              {  x,  y  };
7. • The LINQ idea (which is actually the Haskell idea): comprehensions desugar into generic combinators. • Any datatype that supports those combinators automatically gets to play along.
8. • Defer comprehensions from ES6. • Jafar and I will present an ES7 proposal for generalized comprehensions.
9. Generator Iterator .prototype .prototype [[Prototype]] generator [[Prototype]] [[Prototype]] ??
10. Iterator.prototype.      zip      filter      map      ...
11. table.keys().map(...)                          .filter(...)   ! //  versus   ! import  {  map,  filter  }  from  "itertools";   filter(map(table.keys(),  ...),  ...);
12. (new  Iterator({      next()  {  ...  }   })).map(...)
13. 5 Jun 14 Resolutions
14. • Agree to defer comprehensions. • No future-proofing placeholder objects (can be added later with low compatibility risk). • May not generalize generator comprehensions since first RHS eagerly evaluated; more work to do.