[feature request] chain from iterable, bind?

[coldfix]

Hey,

one composition that seems to be missing is chaining iterators when the list of iterators is given as an iterator - similar to itertools.chain.from_iterable in python.

E.g. something like this:

function chain_from_iterable(iter, gen, state)
  return chain(table.unpack(totable(iter, gen, state))
--return reduce(chain, {}, iter, gen, state)
end

But of course without going through the intermediate table (or doing the reduction) - which might be large, time consuming or even infinite.

IMO, chain_from_iterable is more natural and can easily be used to implement chain = function(...) return chain_from_iterable({...}) end, but not the other way round.

list comprehensions

with this function, you can implement the bind operation of monads (i.e. Haskell's >>=) like this:

function bind(x, f)
  return chain_from_iterable(map(f, x))
end
getmetatable((wrap({}))).__index.bind = bind

which allows to write something like list comprehensions

function even(n) return n%2 == 0 end

range(10):filter(even):bind(function(a) return
  range(a):map(function(b) return
    a*b
  end)
end):each(print)

corresponding to a generator expression such as this:

(a*b
 for a in range(10)
 if a%2 == 0
 for b in range(a))