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Sam Westrick @shwestrick.bsky.social · 19/06/2025

taking a go at oxcaml today first attempt: let's try a parallel reduction. Idea is to compute the "sum" of [f(lo), ..., f(hi-1)] in parallel, combining via some associative function g (and corresponding zero element, z).

a picture of the following OxCaml code:

let rec reduce _par grain g z (lo, hi) f =
let n = hi - lo in
if n <= grain then
let rec loop acc i =
if i >= hi then acc else loop (g acc (f i)) (i+1)
in
loop z lo
else
let mid = lo + n / 2 in
let l, r = Parallel.fork_join2 _par
(fun _par -> reduce _par grain g z (lo, mid) f)
(fun _par -> reduce _par grain g z (mid, hi) f)
in
g l r

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Sam Westrick @shwestrick.bsky.social · 19/06/2025

however, this doesn't typecheck. We get an error that the return value of the reduction in the recursive call is contended.

dune error message. the entire `reduce ...` recursive call is highlighted with an error: "this value is contended but expected to be uncontended"

130 0

Sam Westrick @shwestrick.bsky.social · 19/06/2025

I believe what's happening here is that the result is being inferred as: 'a @ contended But why?

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