Monadic comprehensions

[tiansivive]

Proposed syntax:

let comprehension 
    = \xs -> yield (x + y) from {
      x <- xs if isEven |> (>5) 
      y <- range 0 x

      then take 5
      then group by y
    } using someMonadImpl

We take advantage of curly blocks to introduce then constructs, similar to haskell list comprehensions

We add if clauses (potentially unless too) to backcall, which get desugared to guards on the results of the backcall

Questions:
Do we need to ensure purity? Do we need to work only with monads?

A then statements could simply desugar to introducing a new code block scoped to all the variables of the current block.
For example:

yield (x + y) from {
  x <- xs
  y <- range 0 x

  then take 5
}

Would desugar to:

yield (x + y) from {
  return take 5 {
     x <- xs
     y <- range 0 x
     return (x,y)
  }
}

Consequently, one can do

yield (x + y) from {
  x <- xs
  y <- range 0 x

  (x', y') <- then take 5

  return (x' +1, y' +1)
}

Which desugars to

yield (x + y) from {
  (x', y') <- take 5 {
     x <- xs
     y <- range 0 x
     return (x,y)
  }
  return (x' +1, y' +1)
}