ReScript v12 RC: PPX auto-rewrite for JSX `@defer` causes curried/uncurried mismatch

Frank-III · February 24, 2026, 2:20am

Hi all — I’m investigating a native PPX rewrite edge case and wanted to confirm whether this is a known compiler limitation/bug.

Context

I am building rescript-solid: GitHub - Frank-III/rescript-solid

We have a native PPX that rewrites:

@show (works reliably)
@defer (problematic when payload is JSX)

Current behavior:

Explicit source works:
- SolidJSX.ppxDefer(() => <p>...</p>)
Auto rewrite of JSX `@defer` can fail:
- @defer { <p>...</p> } → rewritten by PPX → with curried/uncurried error

Isolated repro

I created an isolated mini project at:

`examples/defer-repro`

Repro file:

open Solid

@jsx.component
let make = () => {
  let (count, _setCount) = createSignal(0)
  <div>
    <p>{string("Probe")}</p>
    {@defer {
      <p>{string("Deferred count: " ++ count()->Int.toString)}</p>
    }}
  </div>
}

Commands

Baseline (pass):

bun run -F solid-examples-defer-repro res:clean && bun run -F solid-examples-defer-repro res:build

Guarded rewrite mode (pass):

bun run -F solid-examples-defer-repro res:clean && RESCRIPT_SHOW_PPX_NATIVE_DEFER=1 bun run -F solid-examples-defer-repro res:build

Full JSX auto-rewrite mode (fails):

bun run -F solid-examples-defer-repro res:clean && RESCRIPT_SHOW_PPX_NATIVE_DEFER=1 RESCRIPT_SHOW_PPX_NATIVE_DEFER_JSX=1 bun run -F solid-examples-defer-repro res:build

Error

At examples/defer-repro/src/DeferRepro.res on the deferred JSX expression:

This function is a curried function where an uncurried function is expected

tsnobip · February 24, 2026, 6:19am

Hmm I don’t understand why it works when it doesn’t return a jsx but it’s likely caused by the fact we changed how functions are encoded in the AST with uncurried mode, so this is likely not correct:

github.com

Frank-III/rescript-solid/blob/main/packages/rescript-show-ppx-native/bin/show_ppx_native.ml#L28


      
            | Some ("1" | "true" | "TRUE" | "yes" | "on") -> true
            | _ -> false
          
          
let lident ~loc txt =
            { loc; txt = Longident.parse txt }
          
          
let unit_pat ~loc = ppat_construct ~loc (lident ~loc "()") None
          
          
let rewrite_defer expr =
            let loc = expr.pexp_loc in
            let thunk = pexp_fun ~loc Nolabel None (unit_pat ~loc) expr in
            let call = pexp_ident ~loc (lident ~loc "SolidJSX.ppxDefer") in
            pexp_apply ~loc call [ (Nolabel, thunk) ]
          
          
let remove_defer_attr expr =
            {
              expr with
              pexp_attributes = remove_attr "defer" expr.pexp_attributes;
            }
          
          
let normalize_defer_for_rewrite expr =

I’ll check how it should be rewritten.