fix(beam): collapse array-literal process-dict ref round-trips in FFI/Emit calls

[dbrattli]

Problem

Fable-BEAM represents F# arrays as process-dictionary refs (fable_utils:new_ref(...)), so any array-typed value handed to an FFI BIF must be read back with erlang:get(...). When an array literal is passed directly to an FFI/Emit binding that derefs its argument — e.g. [<Emit("maps:from_list(erlang:get($0))")>] — the compiler emits a pointless round-trip:

maps:from_list(erlang:get(fable_utils:new_ref([{<<"content-type">>, <<"text/html">>}])))

It allocates a process-dict ref on every call for an immutable literal that never needed mutable-array semantics (and leaks a never-erased process-dict entry). A hand-written handler just writes maps:from_list([{<<"content-type">>, <<"text/html">>}]).

Reported by a consumer migrating Fable.Actor's timeflies-beam example to the typed Cowboy API.

Approach

fable_utils:new_ref(X) stores X under a fresh inline reference and returns it; an immediately-applied erlang:get reads it straight back, so get(new_ref(X)) ≡ X.

This is recognised on the Beam AST in simplifyArrayRefDerefs (Fable2Beam.fs) when an Emit node is built: when an argument is an inline new_ref(list) and every occurrence of its $N placeholder in the (short, authored) macro template is deref-wrapped (erlang:get($N) / get($N)), the deref is stripped from the template and the underlying list is passed directly.

Gating on the argument's AST shape — rather than scanning rendered Erlang with balanced-paren matching — keeps it robust:

• A ref-typed variable (a bound/mutable array) is left untouched, so its deref is preserved (verified: maps:from_list(erlang:get(Arr))).
• The "every occurrence wrapped" check means macros like put($0, get($0) + 1) (where $0 also appears unwrapped) are never miscompiled.
• Lowering array literals to plain lists at NewArray was rejected: it would break every binding that derefs with erlang:get($0) (you'd get erlang:get([...]) → undefined).

This covers user FFI bindings and Fable's own deref-based replacements (maps:keys/values/to_list, Array.sum/min/max/contains, Set.ofArray, Map.ofArray, …) when given literals. Generated output now matches a hand-written handler:

maps:from_list([{<<"content-type"/utf8>>, <<"text/html"/utf8>>}])

Changes

• Fable2Beam.fs — added simplifyArrayRefDerefs, applied to the args/macro when constructing Emit nodes.
• tests/Beam/InteropTests.fs — regression test feeding a literal array to a deref-style Emit binding.
• FABLE-BEAM.md — documented the behavior.

Testing

Full BEAM suite passes: 2443 passed, 0 failed (including the new test). Quicktest-verified all paths: lists:sum([1,2,3]), maps:from_list([{...},{...}]), and a bound+mutated array still emitting maps:from_list(erlang:get(Arr)).

🤖 Generated with Claude Code