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[Beam] Fix try/catch warnings and reraise unbound variable bug #4392

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[Beam] Fix try/catch warnings and reraise unbound variable bug #4392

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bd6b31a
[Beam] Fix try/catch warnings and reraise unbound variable bug
dbrattli Mar 11, 2026
678ae9b
[Beam] Add changelog entries for try/catch and reraise fixes
dbrattli Mar 11, 2026
c52d235
[Beam] Fix Seq.toList on strings by routing through fable_utils:to_list
dbrattli Mar 11, 2026
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6 changes: 6 additions & 0 deletions src/Fable.Cli/CHANGELOG.md
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Expand Up @@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0

## Unreleased

### Fixed

* [Beam] Fix unused term warning in try/catch when exception variable is not referenced (by @dbrattli)
* [Beam] Fix "no effect" warning for pure BIF calls (`self/0`, `node/0`) in non-final block positions (by @dbrattli)
* [Beam] Fix `reraise()` generating unbound `MatchValue` variable — use raw Erlang reason variable for re-throw (by @dbrattli)

## 5.0.0-rc.3 - 2026-03-10

### Added
Expand Down
6 changes: 6 additions & 0 deletions src/Fable.Compiler/CHANGELOG.md
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Expand Up @@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0

## Unreleased

### Fixed

* [Beam] Fix unused term warning in try/catch when exception variable is not referenced (by @dbrattli)
* [Beam] Fix "no effect" warning for pure BIF calls (`self/0`, `node/0`) in non-final block positions (by @dbrattli)
* [Beam] Fix `reraise()` generating unbound `MatchValue` variable — use raw Erlang reason variable for re-throw (by @dbrattli)

## 5.0.0-rc.10 - 2026-03-10

### Added
Expand Down
35 changes: 20 additions & 15 deletions src/Fable.Transforms/Beam/ErlangPrinter.fs
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Expand Up @@ -3,20 +3,25 @@ module Fable.Transforms.ErlangPrinter
open Fable.AST.Beam
open Fable.Transforms

/// Strip bare `ok` atoms from non-final positions in expression lists.
/// These are stray unit values (F# unit → Erlang `ok`) that have no side effects.
let private stripStrayOk (exprs: ErlExpr list) =
/// Strip expressions with no side effects from non-final positions in expression lists.
/// This removes stray unit values (F# unit → Erlang `ok`), standalone variables,
/// literals, and known pure BIF calls (e.g. `self()`, `node()`) that would otherwise
/// produce "has no effect" warnings from the Erlang compiler.
let private stripNoEffect (exprs: ErlExpr list) =
match exprs with
| []
| [ _ ] -> exprs
| _ ->
let isOkAtom =
let isNoEffect =
function
| Literal(AtomLit(Atom "ok")) -> true
| Literal _ -> true
| Emit("ok", []) -> true
| Variable _ -> true
| Call(None, ("self" | "node"), []) -> true
| Call(Some "erlang", ("self" | "node"), []) -> true
| _ -> false

let nonFinal = exprs.[.. exprs.Length - 2] |> List.filter (not << isOkAtom)
let nonFinal = exprs.[.. exprs.Length - 2] |> List.filter (not << isNoEffect)
nonFinal @ [ exprs.[exprs.Length - 1] ]

module Output =
Expand Down Expand Up @@ -258,7 +263,7 @@ module Output =
sb.Append(") ->") |> ignore
sb.AppendLine() |> ignore

let body = stripStrayOk clause.Body
let body = stripNoEffect clause.Body

body
|> List.iteri (fun j bodyExpr ->
Expand Down Expand Up @@ -296,7 +301,7 @@ module Output =
sb.Append(") ->") |> ignore
sb.AppendLine() |> ignore

let body = stripStrayOk clause.Body
let body = stripNoEffect clause.Body

body
|> List.iteri (fun j bodyExpr ->
Expand Down Expand Up @@ -340,7 +345,7 @@ module Output =
sb.Append(" ->") |> ignore
sb.AppendLine() |> ignore

let caseBody = stripStrayOk clause.Body
let caseBody = stripNoEffect clause.Body

caseBody
|> List.iteri (fun j bodyExpr ->
Expand Down Expand Up @@ -371,7 +376,7 @@ module Output =
// Wrap multi-expression blocks in begin...end to avoid
// comma-separated expressions being misinterpreted as
// separate function call arguments
let filtered = stripStrayOk exprs
let filtered = stripNoEffect exprs
let needsBeginEnd = filtered.Length > 1

if needsBeginEnd then
Expand Down Expand Up @@ -406,7 +411,7 @@ module Output =
| TryCatch(body, catchVar, catchBody, after) ->
sb.AppendLine("try") |> ignore

let tryBody = stripStrayOk body
let tryBody = stripNoEffect body

tryBody
|> List.iteri (fun i bodyExpr ->
Expand All @@ -425,7 +430,7 @@ module Output =
writeIndent ()
sb.AppendLine($" _:%s{catchVar} ->") |> ignore

let catchBody' = stripStrayOk catchBody
let catchBody' = stripNoEffect catchBody

catchBody'
|> List.iteri (fun i bodyExpr ->
Expand Down Expand Up @@ -498,7 +503,7 @@ module Output =
sb.Append(" ->") |> ignore
sb.AppendLine() |> ignore

let caseBody = stripStrayOk clause.Body
let caseBody = stripNoEffect clause.Body

caseBody
|> List.iteri (fun j bodyExpr ->
Expand All @@ -525,7 +530,7 @@ module Output =
sb.Append(" ->") |> ignore
sb.AppendLine() |> ignore

let afterBody = stripStrayOk bodyExprs
let afterBody = stripNoEffect bodyExprs

afterBody
|> List.iteri (fun j bodyExpr ->
Expand Down Expand Up @@ -573,7 +578,7 @@ module Output =
sb.Append(" ->") |> ignore
sb.AppendLine() |> ignore

let topBody = stripStrayOk clause.Body
let topBody = stripNoEffect clause.Body

topBody
|> List.iteri (fun i bodyExpr ->
Expand Down
12 changes: 12 additions & 0 deletions src/Fable.Transforms/Beam/Fable2Beam.Util.fs
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Expand Up @@ -74,6 +74,12 @@ let rec containsIdentRef (name: string) (expr: Expr) : bool =
|| match baseCall with
| Some e -> containsIdentRef name e
| None -> false
| Extended(kind, _) ->
match kind with
| Throw(Some e, _) -> containsIdentRef name e
| Throw(None, _)
| Debugger
| Curry _ -> false
| _ -> false

/// Check if an identifier is captured inside a closure (Lambda/Delegate) within the expression.
Expand Down Expand Up @@ -128,6 +134,12 @@ let isCapturedInClosure (name: string) (expr: Expr) : bool =
|| match baseCall with
| Some e -> check inClosure e
| None -> false
| Extended(kind, _) ->
match kind with
| Throw(Some e, _) -> check inClosure e
| Throw(None, _)
| Debugger
| Curry _ -> false
| _ -> false

check false expr
Expand Down
141 changes: 80 additions & 61 deletions src/Fable.Transforms/Beam/Fable2Beam.fs
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Expand Up @@ -52,6 +52,7 @@ type Context =
CtorFieldExprs: Map<string, Beam.ErlExpr> // field name -> Erlang expr during constructor
ClassFieldPrefix: bool // When true, NewRecord uses "field_" prefix for map keys (for explicit val field class ctors)
CtorParamNames: Set<string> // Constructor parameter names (stored as class fields)
CatchReasonVar: (string * string) option // (catch ident name, Erlang reason var name) for reraise support
}

/// Check if an entity ref refers to an interface type
Expand Down Expand Up @@ -759,88 +760,99 @@ let rec transformExpr (com: IBeamCompiler) (ctx: Context) (expr: Expr) : Beam.Er
let reasonVar = $"Exn_reason_%d{ctr}"
let identVar = capitalizeFirst ident.Name

let ctx' = { ctx with LocalVars = ctx.LocalVars.Add(ident.Name) }
let ctx' =
{ ctx with
LocalVars = ctx.LocalVars.Add(ident.Name)
CatchReasonVar = Some(ident.Name, reasonVar)
}

let erlCatchBody = transformExpr com ctx' catchExpr

let catchBodyExprs =
match erlCatchBody with
| Beam.ErlExpr.Block es -> es
| e -> [ e ]

let reasonRef = Beam.ErlExpr.Variable reasonVar
// Only generate the exception wrapping/binding when the catch body
// actually references the exception identifier. This avoids unused
// term warnings from the Erlang compiler.
if containsIdentRef ident.Name catchExpr then
let reasonRef = Beam.ErlExpr.Variable reasonVar

let formatExpr =
Beam.ErlExpr.Call(
None,
"iolist_to_binary",
[
Beam.ErlExpr.Call(
Some "io_lib",
"format",
[
Beam.ErlExpr.Call(
None,
"binary_to_list",
[ Beam.ErlExpr.Literal(Beam.ErlLiteral.StringLit "~p") ]
)
Beam.ErlExpr.List [ reasonRef ]
]
)
]
)

let messageExpr =
Beam.ErlExpr.Case(
reasonRef,
[
{
Pattern = Beam.PWildcard
Guard = [ Beam.ErlExpr.Call(None, "is_binary", [ reasonRef ]) ]
Body = [ reasonRef ]
}
{
Pattern = Beam.PWildcard
Guard = []
Body = [ formatExpr ]
}
]
)
let formatExpr =
Beam.ErlExpr.Call(
None,
"iolist_to_binary",
[
Beam.ErlExpr.Call(
Some "io_lib",
"format",
[
Beam.ErlExpr.Call(
None,
"binary_to_list",
[ Beam.ErlExpr.Literal(Beam.ErlLiteral.StringLit "~p") ]
)
Beam.ErlExpr.List [ reasonRef ]
]
)
]
)

// If reason is already a map (F# exception) or reference (class inheriting exn), use it directly.
// Otherwise wrap in #{message => ...} for .Message access.
let bindIdent =
Beam.ErlExpr.Match(
Beam.PVar identVar,
let messageExpr =
Beam.ErlExpr.Case(
reasonRef,
[
{
Pattern = Beam.PWildcard
Guard = [ Beam.ErlExpr.Call(None, "is_map", [ reasonRef ]) ]
Body = [ reasonRef ]
}
{
Pattern = Beam.PWildcard
Guard = [ Beam.ErlExpr.Call(None, "is_reference", [ reasonRef ]) ]
Guard = [ Beam.ErlExpr.Call(None, "is_binary", [ reasonRef ]) ]
Body = [ reasonRef ]
}
{
Pattern = Beam.PWildcard
Guard = []
Body =
[
Beam.ErlExpr.Map
[
Beam.ErlExpr.Literal(Beam.ErlLiteral.AtomLit(Beam.Atom "message")),
messageExpr
]
]
Body = [ formatExpr ]
}
]
)
)

Beam.ErlExpr.TryCatch(bodyExprs, reasonVar, [ bindIdent ] @ catchBodyExprs, afterExprs)
// If reason is already a map (F# exception) or reference (class inheriting exn), use it directly.
// Otherwise wrap in #{message => ...} for .Message access.
let bindIdent =
Beam.ErlExpr.Match(
Beam.PVar identVar,
Beam.ErlExpr.Case(
reasonRef,
[
{
Pattern = Beam.PWildcard
Guard = [ Beam.ErlExpr.Call(None, "is_map", [ reasonRef ]) ]
Body = [ reasonRef ]
}
{
Pattern = Beam.PWildcard
Guard = [ Beam.ErlExpr.Call(None, "is_reference", [ reasonRef ]) ]
Body = [ reasonRef ]
}
{
Pattern = Beam.PWildcard
Guard = []
Body =
[
Beam.ErlExpr.Map
[
Beam.ErlExpr.Literal(Beam.ErlLiteral.AtomLit(Beam.Atom "message")),
messageExpr
]
]
}
]
)
)

Beam.ErlExpr.TryCatch(bodyExprs, reasonVar, [ bindIdent ] @ catchBodyExprs, afterExprs)
else
Beam.ErlExpr.TryCatch(bodyExprs, reasonVar, catchBodyExprs, afterExprs)
| None, [] ->
// No catch handler and no finalizer
erlBody
Expand Down Expand Up @@ -1156,8 +1168,14 @@ let rec transformExpr (com: IBeamCompiler) (ctx: Context) (expr: Expr) : Beam.Er
| Extended(kind, _range) ->
match kind with
| Throw(Some exprArg, _typ) ->
let erlExpr = transformExpr com ctx exprArg
Beam.ErlExpr.Call(Some "erlang", "error", [ erlExpr ])
// For reraise: if the thrown expression references the catch ident,
// use the raw Erlang reason variable to preserve the original exception.
match exprArg, ctx.CatchReasonVar with
| IdentExpr ident, Some(catchIdentName, reasonVar) when ident.Name = catchIdentName ->
Beam.ErlExpr.Call(Some "erlang", "error", [ Beam.ErlExpr.Variable reasonVar ])
| _ ->
let erlExpr = transformExpr com ctx exprArg
Beam.ErlExpr.Call(Some "erlang", "error", [ erlExpr ])
| Throw(None, _typ) ->
// Re-raise (should not normally happen outside catch context)
Beam.ErlExpr.Call(
Expand Down Expand Up @@ -3355,6 +3373,7 @@ let transformFile (com: Fable.Compiler) (file: File) : Beam.ErlModule =
CtorFieldExprs = Map.empty
ClassFieldPrefix = false
CtorParamNames = Set.empty
CatchReasonVar = None
}

let ctorNameRegistry = System.Collections.Generic.Dictionary<string, string>()
Expand Down
4 changes: 4 additions & 0 deletions src/Fable.Transforms/Beam/Replacements.fs
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Expand Up @@ -1887,6 +1887,10 @@ let private seqModule
=
match info.CompiledName, args with
| "Cast", [ arg ] -> Some arg
| "ToList", [ arg ] ->
// Use fable_utils:to_list which handles binaries (strings), plain lists,
// ref-wrapped arrays, enumerator maps, and lazy seqs.
emitExpr r t [ arg ] "fable_utils:to_list($0)" |> Some
| "ToArray", [ arg ] ->
// seq:to_array already returns a ref-wrapped array, don't double-wrap
Helper.LibCall(com, "seq", "to_array", t, [ arg ], info.SignatureArgTypes, ?loc = r)
Expand Down
6 changes: 6 additions & 0 deletions tests/Beam/SeqTests.fs
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Expand Up @@ -1034,3 +1034,9 @@ let ``test Seq.except works with various types`` () =
Seq.except [(1, 2)] [(1, 2)] |> Seq.isEmpty |> equal true
Seq.except [|49|] [|7; 49|] |> Seq.last |> equal 7
Seq.except [{ Bar= "test" }] [{ Bar = "test" }] |> Seq.isEmpty |> equal true

[<Fact>]
let ``test Seq.toList on string works`` () =
let text = "ABC"
let chars = text |> Seq.toList
chars |> List.length |> equal 3
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