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refactor(beam): give string x and x.ToString() one dispatch #4816

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Merged
dbrattli merged 1 commit into from
Jul 18, 2026
+35 −36
Merged

refactor(beam): give string x and x.ToString() one dispatch #4816

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33 changes: 33 additions & 0 deletions src/Fable.Transforms/Beam/Prelude.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -520,6 +520,39 @@ module Chars =
| Some _ -> Char
| None -> e.Type

/// Conversion of a value to its `string` form, dispatched on the static type.
module ToString =
open Fable.AST
open Fable.AST.Fable
open Fable.Transforms
open Fable.Transforms.Replacements.Util

/// The `ToString` of a value whose type the compiler can still see.
///
/// Two replacement tables reach this: `string x` as an operator, and `x.ToString()` as a
/// conversion. They must not diverge — a fix applied to one spelling and not the other is
/// invisible until someone happens to test the neglected one — so both call here.
///
/// A char is peeled out of its boxing cast first. `box c |> string` arrives as a char under a
/// cast to `obj` and would otherwise fall through to `fable_convert:to_string`, which sees only
/// the integer and prints the codepoint. Every other type is unaffected, since `tryAsChar` only
/// ever looks through a box around a char.
let toStringByType (com: ICompiler) (r: SourceLocation option) (t: Type) (arg: Expr) =
let arg = Chars.tryAsChar arg |> Option.defaultValue arg

match arg.Type with
| Type.String -> Some arg
| Type.Char -> emitExpr r t [ arg ] "<<($0)/utf8>>" |> Some
| Type.Number(kind, _) ->
match kind with
| Decimal -> Helper.LibCall(com, "fable_decimal", "to_string", t, [ arg ], ?loc = r) |> Some
| Float16
| Float32
| Float64 -> Helper.LibCall(com, "fable_convert", "to_string", t, [ arg ], ?loc = r) |> Some
| _ -> emitExpr r t [ arg ] "integer_to_binary($0)" |> Some
| Type.Boolean -> emitExpr r t [ arg ] "atom_to_binary($0)" |> Some
| _ -> Helper.LibCall(com, "fable_convert", "to_string", t, [ arg ], ?loc = r) |> Some

/// Fixed-width integer semantics. Erlang integers are arbitrary precision and never
/// overflow, so operations that can leave the width of a .NET sized integer are routed
/// through the `fable_int` runtime module to truncate them back (two's complement).
Expand Down
38 changes: 2 additions & 36 deletions src/Fable.Transforms/Beam/Replacements.fs
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Expand Up @@ -392,25 +392,7 @@ let private operators
|> Some
| _ -> Helper.LibCall(com, "fable_decimal", "from_int", _t, [ arg ], ?loc = r) |> Some
| _ -> Helper.LibCall(com, "fable_decimal", "from_int", _t, [ arg ], ?loc = r) |> Some
| "ToString", [ arg ] ->
// `box c |> string` arrives as a char under a boxing cast to `obj`, and would otherwise fall
// through to `fable_convert:to_string`, which sees only the integer and prints the codepoint.
// Peeling the cast puts it back on the `Type.Char` branch below; every other type is
// unaffected, since `tryAsChar` only ever looks through a box around a char.
let arg = Chars.tryAsChar arg |> Option.defaultValue arg

match arg.Type with
| Type.String -> Some arg
| Type.Char -> emitExpr r _t [ arg ] "<<($0)/utf8>>" |> Some
| Type.Number(kind, _) ->
match kind with
| Decimal -> Helper.LibCall(com, "fable_decimal", "to_string", _t, [ arg ], ?loc = r) |> Some
| Float16
| Float32
| Float64 -> Helper.LibCall(com, "fable_convert", "to_string", _t, [ arg ], ?loc = r) |> Some
| _ -> emitExpr r _t [ arg ] "integer_to_binary($0)" |> Some
| Type.Boolean -> emitExpr r _t [ arg ] "atom_to_binary($0)" |> Some
| _ -> Helper.LibCall(com, "fable_convert", "to_string", _t, [ arg ], ?loc = r) |> Some
| "ToString", [ arg ] -> ToString.toStringByType com r _t arg
| "ToChar", [ arg ] ->
match arg.Type with
| Type.String -> emitExpr r _t [ arg ] "binary:first($0)" |> Some
Expand Down Expand Up @@ -1418,23 +1400,7 @@ let private conversions
| [ a ] -> a
| _ -> Value(Null Type.Unit, None)

// `box c |> string` reaches here as a char under a boxing cast to `obj`. Peeling the cast
// puts it back on the `Type.Char` branch below, instead of falling through to
// `fable_convert:to_string`, which sees only the integer and prints the codepoint.
let arg = Chars.tryAsChar arg |> Option.defaultValue arg

match arg.Type with
| Type.String -> Some arg
| Type.Char -> emitExpr r t [ arg ] "<<($0)/utf8>>" |> Some
| Type.Number(kind, _) ->
match kind with
| Decimal -> Helper.LibCall(com, "fable_decimal", "to_string", t, [ arg ], ?loc = r) |> Some
| Float16
| Float32
| Float64 -> Helper.LibCall(com, "fable_convert", "to_string", t, [ arg ], ?loc = r) |> Some
| _ -> emitExpr r t [ arg ] "integer_to_binary($0)" |> Some
| Type.Boolean -> emitExpr r t [ arg ] "atom_to_binary($0)" |> Some
| _ -> Helper.LibCall(com, "fable_convert", "to_string", t, [ arg ], ?loc = r) |> Some
ToString.toStringByType com r t arg
// char(x) → Erlang integers represent chars
| "ToChar", [ arg ] ->
match arg.Type with
Expand Down
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