OneShot Monad Trick - Githubissues

cite this trick and provide code examples in GHC, the note is in GHC.Utils.Monad.hs
describe exactly why this works (I think this was in Joachim's thesis?)
tie it to a GHC specific system, i.e., the occurence analyzer

Here is some of the Note to wet your appetite:

{- Note [The one-shot state monad trick]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Summary: many places in GHC use a state monad, and we really want those
functions to be eta-expanded (#18202).

The problem
~~~~~~~~~~~
Consider
    newtype M a = MkM (State -> (State, a))

    instance Monad M where
       mf >>= k = MkM (\s -> case mf  of MkM f  ->
                             case f s of (s',r) ->
                             case k r of MkM g  ->
                             g s')

    fooM :: Int -> M Int
    fooM x = g y >>= \r -> h r
      where
        y = expensive x

Now suppose you say (repeat 20 (fooM 4)), where
  repeat :: Int -> M Int -> M Int
performs its argument n times.  You would expect (expensive 4) to be
evaluated only once, not 20 times.  So foo should have arity 1 (not 2);
it should look like this (modulo casts)

  fooM x = let y = expensive x in
           \s -> case g y of ...

But creating and then repeating, a monadic computation is rare.  If you
/aren't/ re-using (M a) value, it's /much/ more efficient to make
foo have arity 2, thus:

  fooM x s = case g (expensive x) of ...

Why more efficient?  Because now foo takes its argument both at once,
rather than one at a time, creating a heap-allocated function closure. See
https://www.joachim-breitner.de/blog/763-Faster_Winter_5__Eta-Expanding_ReaderT
for a very good explanation of the issue which led to these optimisations
into GHC.

The trick
~~~~~~~~~
...

haskellfoundation / hs-opt-handbook.github.io

OneShot Monad Trick #53