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Resumable state machines: Don't capture definitions by "builder@" prefix. EXPERIMENTAL #19482

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Resumable state machines: Don't capture definitions by "builder@" prefix. EXPERIMENTAL #19482

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fe2e2d2
Simplify state machine binding detection logic
majocha Mar 23, 2026
858ccfc
Merge branch 'main' into builder-resumable
majocha Mar 25, 2026
e8521de
eliminate the elimination
majocha Apr 1, 2026
9b92de4
Merge branch 'main' into builder-resumable
majocha Apr 1, 2026
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12 changes: 1 addition & 11 deletions src/Compiler/Optimize/LowerStateMachines.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -102,15 +102,7 @@ let RepresentBindingAsStateVar g (bind: Binding) (resBody: StateMachineConversio
let isExpandVar g (v: Val) =
isReturnsResumableCodeTy g v.TauType

// We allow a prefix of bindings prior to the state machine, e.g.
// task { .. }
// becomes
// let builder@ = task
// ....
let isStateMachineBindingVar g (v: Val) =
isExpandVar g v ||
(let nm = v.LogicalName
(nm.StartsWithOrdinal("builder@") || v.IsMemberThisVal))
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@T-Gro T-Gro Apr 1, 2026

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Do we know why it was needed before?
Are the recent state machine bug fixes the reason why we no longer need this special casing?

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@majocha majocha Apr 1, 2026

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Are the recent state machine bug fixes the reason why we no longer need this special casing?

No, this is completely independent. I did a checkout of old commit and applied this fix to check it.

Do we know why it was needed before?

Ha, this is not entirely clear. That's why I'm a bit wary about this.

let isStateMachineBindingVar g (v: Val) = isExpandVar g v

type env =
{
Expand Down Expand Up @@ -361,7 +353,6 @@ type LowerStateMachine(g: TcGlobals, outerResumableCodeDefns: ValMap<Expr>) =
TryReduceExpr env f (args2 @ args) (fun f2 -> remake (Expr.App (f2, _fty, _tyargs, args2, _m)))

| _ ->
//let (env, expr) = BindResumableCodeDefinitions env expr
match TryReduceApp env expr args with
| Some expandedExpr ->
if sm_verbose then printfn "reduction = %A, args = %A --> %A..." expr args expandedExpr
Expand Down Expand Up @@ -408,7 +399,6 @@ type LowerStateMachine(g: TcGlobals, outerResumableCodeDefns: ValMap<Expr>) =
// Detect a state machine with a single method override
[<return: Struct>]
let (|ExpandedStateMachineInContext|_|) inputExpr =
// All expanded resumable code state machines e.g. 'task { .. }' begin with a bind of @builder or 'defn'
// Seed the env with any expand-var definitions from outer scopes (e.g. across lambda boundaries)
let initialEnv = { env.Empty with ResumableCodeDefns = outerResumableCodeDefns }
let env, expr = BindResumableCodeDefinitions initialEnv inputExpr
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