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MangelMaxime merged 4 commits into from
Jun 27, 2026
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feat(all): add Compiler.is* target detection flags for conditional branching #4692

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021419c
feat(all): add `Compiler.is*` target detection flags for conditional …
MangelMaxime Jun 26, 2026
af50fc3
feat: add `Compiler.isDotnet`
MangelMaxime Jun 26, 2026
bfd572c
test: fix
MangelMaxime Jun 26, 2026
64dd686
fix: update Fable.Core metadata
MangelMaxime Jun 27, 2026
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24 changes: 24 additions & 0 deletions src/Fable.Core/Fable.Core.Util.fs
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Expand Up @@ -69,3 +69,27 @@ module Compiler =
/// In watch compilations, indicates if the file is being recompiled
/// not because of a direct change, but because a dependency has changed
let triggeredByDependency: bool = false

/// Indicates if code is running on .NET (not compiled by Fable)
let isDotnet: bool = true

/// Indicates if Fable is compiling to JavaScript
let isJavaScript: bool = false

/// Indicates if Fable is compiling to TypeScript
let isTypeScript: bool = false

/// Indicates if Fable is compiling to Python
let isPython: bool = false

/// Indicates if Fable is compiling to Dart
let isDart: bool = false

/// Indicates if Fable is compiling to Rust
let isRust: bool = false

/// Indicates if Fable is compiling to PHP
let isPhp: bool = false

/// Indicates if Fable is compiling to Erlang/BEAM
let isBeam: bool = false
24 changes: 24 additions & 0 deletions src/Fable.Transforms/Beam/Replacements.fs
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Expand Up @@ -5478,6 +5478,30 @@ let tryCall
match info.CompiledName with
| "get_InvariantCulture" -> emitExpr r t [] "undefined" |> Some
| _ -> None
| "Fable.Core.Compiler" ->
match info.CompiledName with
| "version" -> makeStrConst Literals.VERSION |> Some
| "majorMinorVersion" ->
try
let m = System.Text.RegularExpressions.Regex.Match(Literals.VERSION, @"^\d+\.\d+")
float m.Value |> makeFloatConst |> Some
with _ ->
"Cannot parse compiler version"
|> addErrorAndReturnNull com ctx.InlinePath r
|> Some
| "debugMode" -> makeBoolConst com.Options.DebugMode |> Some
| "typedArrays" -> makeBoolConst com.Options.TypedArrays |> Some
| "extension" -> makeStrConst com.Options.FileExtension |> Some
| "triggeredByDependency" -> makeBoolConst com.Options.TriggeredByDependency |> Some
| "isDotnet" -> makeBoolConst false |> Some
| "isJavaScript" -> makeBoolConst (com.Options.Language = JavaScript) |> Some
| "isTypeScript" -> makeBoolConst (com.Options.Language = TypeScript) |> Some
| "isPython" -> makeBoolConst (com.Options.Language = Python) |> Some
| "isDart" -> makeBoolConst (com.Options.Language = Dart) |> Some
| "isRust" -> makeBoolConst (com.Options.Language = Rust) |> Some
| "isPhp" -> makeBoolConst (com.Options.Language = Php) |> Some
| "isBeam" -> makeBoolConst (com.Options.Language = Beam) |> Some
| _ -> None
| "Fable.Core.BeamInterop" ->
match info.CompiledName, args with
| Naming.StartsWith "import" suffix, _ ->
Expand Down
8 changes: 8 additions & 0 deletions src/Fable.Transforms/Dart/Replacements.fs
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Expand Up @@ -732,6 +732,14 @@ let fableCoreLib (com: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Exp
| "typedArrays" -> makeBoolConst com.Options.TypedArrays |> Some
| "extension" -> makeStrConst com.Options.FileExtension |> Some
| "triggeredByDependency" -> makeBoolConst com.Options.TriggeredByDependency |> Some
| "isDotnet" -> makeBoolConst false |> Some
| "isJavaScript" -> makeBoolConst (com.Options.Language = JavaScript) |> Some
| "isTypeScript" -> makeBoolConst (com.Options.Language = TypeScript) |> Some
| "isPython" -> makeBoolConst (com.Options.Language = Python) |> Some
| "isDart" -> makeBoolConst (com.Options.Language = Dart) |> Some
| "isRust" -> makeBoolConst (com.Options.Language = Rust) |> Some
| "isPhp" -> makeBoolConst (com.Options.Language = Php) |> Some
| "isBeam" -> makeBoolConst (com.Options.Language = Beam) |> Some
| _ -> None
| Naming.StartsWith "Fable.Core.Dart" rest, _ ->
match rest with
Expand Down
8 changes: 8 additions & 0 deletions src/Fable.Transforms/Python/Replacements.fs
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Expand Up @@ -954,6 +954,14 @@ let fableCoreLib (com: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Exp
| "debugMode" -> makeBoolConst com.Options.DebugMode |> Some
| "typedArrays" -> makeBoolConst com.Options.TypedArrays |> Some
| "extension" -> makeStrConst com.Options.FileExtension |> Some
| "isDotnet" -> makeBoolConst false |> Some
| "isJavaScript" -> makeBoolConst (com.Options.Language = JavaScript) |> Some
| "isTypeScript" -> makeBoolConst (com.Options.Language = TypeScript) |> Some
| "isPython" -> makeBoolConst (com.Options.Language = Python) |> Some
| "isDart" -> makeBoolConst (com.Options.Language = Dart) |> Some
| "isRust" -> makeBoolConst (com.Options.Language = Rust) |> Some
| "isPhp" -> makeBoolConst (com.Options.Language = Php) |> Some
| "isBeam" -> makeBoolConst (com.Options.Language = Beam) |> Some
| _ -> None
| "Fable.Core.Py", ("python" | "expr_python" as meth) ->
let isStatement = meth <> "expr_python"
Expand Down
8 changes: 8 additions & 0 deletions src/Fable.Transforms/Replacements.fs
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Expand Up @@ -1089,6 +1089,14 @@ let fableCoreLib (com: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Exp
| "typedArrays" -> makeBoolConst com.Options.TypedArrays |> Some
| "extension" -> makeStrConst com.Options.FileExtension |> Some
| "triggeredByDependency" -> makeBoolConst com.Options.TriggeredByDependency |> Some
| "isDotnet" -> makeBoolConst false |> Some
| "isJavaScript" -> makeBoolConst (com.Options.Language = JavaScript) |> Some
| "isTypeScript" -> makeBoolConst (com.Options.Language = TypeScript) |> Some
| "isPython" -> makeBoolConst (com.Options.Language = Python) |> Some
| "isDart" -> makeBoolConst (com.Options.Language = Dart) |> Some
| "isRust" -> makeBoolConst (com.Options.Language = Rust) |> Some
| "isPhp" -> makeBoolConst (com.Options.Language = Php) |> Some
| "isBeam" -> makeBoolConst (com.Options.Language = Beam) |> Some
| _ -> None
| "Fable.Core.JS", ("js" | "expr_js" as meth) ->
let isStatement = meth <> "expr_js"
Expand Down
8 changes: 8 additions & 0 deletions src/Fable.Transforms/Rust/Replacements.fs
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Expand Up @@ -741,6 +741,14 @@ let fableCoreLib (com: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Exp
| "debugMode" -> makeBoolConst com.Options.DebugMode |> Some
| "typedArrays" -> makeBoolConst com.Options.TypedArrays |> Some
| "extension" -> makeStrConst com.Options.FileExtension |> Some
| "isDotnet" -> makeBoolConst false |> Some
| "isJavaScript" -> makeBoolConst (com.Options.Language = JavaScript) |> Some
| "isTypeScript" -> makeBoolConst (com.Options.Language = TypeScript) |> Some
| "isPython" -> makeBoolConst (com.Options.Language = Python) |> Some
| "isDart" -> makeBoolConst (com.Options.Language = Dart) |> Some
| "isRust" -> makeBoolConst (com.Options.Language = Rust) |> Some
| "isPhp" -> makeBoolConst (com.Options.Language = Php) |> Some
| "isBeam" -> makeBoolConst (com.Options.Language = Beam) |> Some
| _ -> None
| "Fable.Core.RustInterop", "op_BangHat" -> List.tryHead args
| "Fable.Core.RustInterop", _ ->
Expand Down
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36 changes: 36 additions & 0 deletions tests/Beam/MiscTests.fs
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Expand Up @@ -3,6 +3,7 @@ module Fable.Tests.Misc
#nowarn "40"

open System
open Fable.Core
open FSharp.UMX
open Util.Testing
open Util2.Extensions
Expand Down Expand Up @@ -1178,3 +1179,38 @@ let ``test Optimized assignment blocks inside try ... with work`` () =
try A.C.Helper.Add5(let mutable x = 2 in let mutable y = 3 in x + y)
with _ -> 1
equal 10 res

[<Fact>]
let ``test Compiler target flags have correct value per target`` () =
equal false Compiler.isJavaScript
equal false Compiler.isTypeScript
equal false Compiler.isPython
equal false Compiler.isDart
equal false Compiler.isRust
#if FABLE_COMPILER_BEAM
equal true Compiler.isBeam
#else
equal false Compiler.isBeam
#endif
#if FABLE_COMPILER
equal false Compiler.isDotnet
#else
equal true Compiler.isDotnet
#endif
equal false (Compiler.isJavaScript || Compiler.isTypeScript)

[<Fact>]
let ``test Compiler target flags eliminate dead branches`` () =
let target =
if Compiler.isJavaScript then "javascript"
elif Compiler.isTypeScript then "typescript"
elif Compiler.isPython then "python"
elif Compiler.isDart then "dart"
elif Compiler.isRust then "rust"
elif Compiler.isBeam then "beam"
else "dotnet"
#if FABLE_COMPILER_BEAM
equal "beam" target
#else
equal "dotnet" target
#endif
32 changes: 32 additions & 0 deletions tests/Dart/src/MiscTests.fs
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@@ -1,6 +1,7 @@
module Fable.Tests.Dart.Misc

open System
open Fable.Core
open Util

let increase (x: int ref) =
Expand Down Expand Up @@ -129,3 +130,34 @@ let tests() =
throwsError "This is invalid (Parameter 'arg')" (fun () ->
invalidArg "arg" "This is invalid"
)

testCase "Compiler target flags have correct value per target" <| fun () ->
equal false Compiler.isJavaScript
equal false Compiler.isTypeScript
equal false Compiler.isPython
#if FABLE_COMPILER_DART
equal true Compiler.isDart
#else
equal false Compiler.isDart
#endif
equal false Compiler.isRust
#if FABLE_COMPILER
equal false Compiler.isDotnet
#else
equal true Compiler.isDotnet
#endif
equal false (Compiler.isJavaScript || Compiler.isTypeScript)

testCase "Compiler target flags eliminate dead branches" <| fun () ->
let target =
if Compiler.isJavaScript then "javascript"
elif Compiler.isTypeScript then "typescript"
elif Compiler.isPython then "python"
elif Compiler.isDart then "dart"
elif Compiler.isRust then "rust"
else "dotnet"
#if FABLE_COMPILER_DART
equal "dart" target
#else
equal "dotnet" target
#endif
12 changes: 12 additions & 0 deletions tests/Integration/Integration/data/conditionalTargetBranching/ConditionalTargetBranching.fs
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@@ -0,0 +1,12 @@
module ConditionalTargetBranching

open Fable.Core

let target =
if Compiler.isJavaScript then "javascript"
elif Compiler.isPython then "python"
else "dotnet"

let targetFromFamily =
if Compiler.isJavaScript || Compiler.isTypeScript then "js-family"
else "other"
3 changes: 3 additions & 0 deletions ...ation/Integration/data/conditionalTargetBranching/ConditionalTargetBranching.jsx.expected
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@@ -0,0 +1,3 @@
export const target = "javascript";

export const targetFromFamily = "js-family";
20 changes: 20 additions & 0 deletions ...Integration/Integration/data/conditionalTargetBranching/conditionalTargetBranching.fsproj
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@@ -0,0 +1,20 @@
<Project Sdk="Microsoft.NET.Sdk">

<PropertyGroup>
<TargetFramework>net10.0</TargetFramework>
<RollForward>Major</RollForward>
<GenerateDocumentationFile>true</GenerateDocumentationFile>
</PropertyGroup>

<ItemGroup>
<Compile Include="ConditionalTargetBranching.fs" />
</ItemGroup>

<ItemGroup>
<!-- Reference the local build since Compiler.is* are not yet in a NuGet release -->
<Reference Include="Fable.Core">
<HintPath>../../../../../src/Fable.Core/bin/Release/netstandard2.0/Fable.Core.dll</HintPath>
</Reference>
</ItemGroup>

</Project>
41 changes: 41 additions & 0 deletions tests/Js/Main/MiscTests.fs
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Expand Up @@ -379,7 +379,7 @@

let mutable mutableValue = 0

let rec recursive1 = delay (fun () -> recursive2())

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This and other recursive references to the object(s) being defined will be checked for initialization-soundness at runtime through the use of a delayed reference. This is because you are defining one or more recursive objects, rather than recursive functions. This warning may be suppressed by using '#nowarn "40"' or '--nowarn:40'.

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This and other recursive references to the object(s) being defined will be checked for initialization-soundness at runtime through the use of a delayed reference. This is because you are defining one or more recursive objects, rather than recursive functions. This warning may be suppressed by using '#nowarn "40"' or '--nowarn:40'.
and recursive2 =
mutableValue <- 5
fun () -> mutableValue <- mutableValue * 2
Expand Down Expand Up @@ -609,6 +609,47 @@
".js"
#endif
Compiler.extension.EndsWith(ext) |> equal true

testCase "isJavaScript || isTypeScript is true for JS family targets" <| fun _ ->
equal true (Compiler.isJavaScript || Compiler.isTypeScript)
equal false (Compiler.isPython || Compiler.isDart || Compiler.isRust || Compiler.isBeam)

testCase "Compiler target flags have correct value per target" <| fun _ ->
#if FABLE_COMPILER_JAVASCRIPT
equal true Compiler.isJavaScript
equal false Compiler.isTypeScript
#endif
#if FABLE_COMPILER_TYPESCRIPT
equal false Compiler.isJavaScript
equal true Compiler.isTypeScript
#endif
equal false Compiler.isPython
equal false Compiler.isDart
equal false Compiler.isRust
#if FABLE_COMPILER
equal false Compiler.isDotnet
#else
equal true Compiler.isDotnet
#endif
equal true (Compiler.isJavaScript || Compiler.isTypeScript)

testCase "Compiler target flags eliminate dead branches" <| fun _ ->
let target =
if Compiler.isJavaScript then "javascript"
elif Compiler.isTypeScript then "typescript"
elif Compiler.isPython then "python"
else "dotnet"
let family =
if Compiler.isJavaScript || Compiler.isTypeScript then "js-family"
else "other"
#if FABLE_COMPILER_JAVASCRIPT
equal "javascript" target
equal "js-family" family
#endif
#if FABLE_COMPILER_TYPESCRIPT
equal "typescript" target
equal "js-family" family
#endif
#endif

testCase "Values of autogenerated functions are not replaced by optimizations" <| fun () -> // See #1583
Expand Down Expand Up @@ -1170,7 +1211,7 @@
| 2 -> x <- "2"
| 3 | 4 -> x <- "3" // Multiple cases are allowed
// | 5 | 6 as j -> x <- string j // This prevents the optimization
| 4 -> x <- "4" // Unreachable cases are removed

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| _ -> x <- "?"
equal "3" x

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23 changes: 23 additions & 0 deletions tests/Python/TestMisc.fs
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Expand Up @@ -531,6 +531,29 @@
let ``test Can access extension for generated files`` () =
Compiler.extension.EndsWith(".py") |> equal true

[<Fact>]
let ``test Compiler target flags have correct value per target`` () =
equal false Compiler.isJavaScript
equal false Compiler.isTypeScript
equal true Compiler.isPython
equal false Compiler.isDart
equal false Compiler.isRust
#if FABLE_COMPILER
equal false Compiler.isDotnet
#else
equal true Compiler.isDotnet
#endif
equal false (Compiler.isJavaScript || Compiler.isTypeScript)

[<Fact>]
let ``test Compiler target flags eliminate dead branches`` () =
let target =
if Compiler.isJavaScript then "javascript"
elif Compiler.isTypeScript then "typescript"
elif Compiler.isPython then "python"
else "dotnet"
equal "python" target

#endif

[<Fact>]
Expand Down Expand Up @@ -1156,7 +1179,7 @@
| 2 -> x <- "2"
| 3 | 4 -> x <- "3" // Multiple cases are allowed
// | 5 | 6 as j -> x <- string j // This prevents the optimization
| 4 -> x <- "4" // Unreachable cases are removed

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| _ -> x <- "?"
equal "3" x

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25 changes: 25 additions & 0 deletions tests/Rust/tests/src/MiscTests.fs
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Expand Up @@ -587,6 +587,31 @@
// let ``Can access extension for generated files`` () =
// Compiler.extension.EndsWith(".js") |> equal true

[<Fact>]
let ``Compiler target flags have correct value per target`` () =
equal false Compiler.isJavaScript
equal false Compiler.isTypeScript
equal false Compiler.isPython
equal false Compiler.isDart
equal true Compiler.isRust
#if FABLE_COMPILER
equal false Compiler.isDotnet
#else
equal true Compiler.isDotnet
#endif
equal false (Compiler.isJavaScript || Compiler.isTypeScript)

[<Fact>]
let ``Compiler target flags eliminate dead branches`` () =
let target =
if Compiler.isJavaScript then "javascript"
elif Compiler.isTypeScript then "typescript"
elif Compiler.isPython then "python"
elif Compiler.isDart then "dart"
elif Compiler.isRust then "rust"
else "dotnet"
equal "rust" target

#endif

[<Fact>]
Expand Down Expand Up @@ -1235,7 +1260,7 @@
| 2 -> x <- "2"
| 3 | 4 -> x <- "3" // Multiple cases are allowed
// | 5 | 6 as j -> x <- string j // This prevents the optimization
| 4 -> x <- "4" // Unreachable cases are removed

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| _ -> x <- "?"
equal "3" x

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