Re: The store monad

[ludo@gnu.org (Ludovic Courtès)]

Today’s monadic news is that monads are “inlined”.  Before, something like:

  (mlet %store-monad ((x y))
    (return (+ 1 y)))

would expand to:

  ((struct-ref %store-monad 0) y
    (lambda (x)
      ((struct-ref %store-monad 1) x)))

With inlining, the above doesn’t expand to references to the
‘%store-monad’ record, and instead directly expands to:

  (store-bind y (lambda (x) (store-return (+ 1 x))))

If in addition ‘store-bind’ and ‘store-return’ are defined with
‘define-inlinable’, then the whole shebang is inlined, allowing for
better optimizations:

--8<---------------cut here---------------start------------->8---
scheme@(guix monads)> ,expand (with-monad %identity-monad (>>= (return 2) (lambda (x) (return (1+ x)))))
$17 = (let ((mvalue (let ((value 2)) value))
      (mproc (lambda (x) (let ((value (#{1+}# x))) value))))
  (mproc mvalue))
scheme@(guix monads)> ,optimize (with-monad %identity-monad (>>= (return 2) (lambda (x) (return (1+ x)))))
$18 = 3
scheme@(guix monads)> ,optimize (mlet* %identity-monad ((x (return 1))(y (return 2))(z (return (+ x y)))) z)
$19 = 3

[...]

scheme@(guix monads)> ,optimize (mlet* %store-monad ((x (return 1))(y (return 2))(z (return (+ x y)))) (return z))
$22 = (lambda (store) (let mresult ((store store)) 3))
--8<---------------cut here---------------end--------------->8---

This significantly reduces the cost of using bind/return all over the
place.

Ludo’.