[Python/Beam] Add F# quotation support (<@ ... @>) (#4398)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dbrattli

src/Fable.AST/Fable.fs

@@ -836,11 +836,13 @@ type Expr =
 
     | Unresolved of expr: UnresolvedExpr * typ: Type * range: SourceLocation option
     | Extended of expr: ExtendedSet * range: SourceLocation option
+    | Quote of quotedExpr: Expr * isTyped: bool * range: SourceLocation option
 
     member this.Type =
         match this with
         | Unresolved(_, t, _) -> t
         | Extended(kind, _) -> kind.Type
+        | Quote _ -> Any
         | Test _ -> Boolean
         | Value(kind, _) -> kind.Type
         | IdentExpr id -> id.Type
@@ -892,6 +894,7 @@ type Expr =
         | Set(_, _, _, _, r)
         | ForLoop(_, _, _, _, _, r)
         | WhileLoop(_, _, r) -> r
+        | Quote(_, _, r) -> r
 
 // module PrettyPrint =
 //     let rec printType (t: Type) = "T" // TODO

src/Fable.Cli/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+* [Python/Beam] Add F# quotation support — construction, pattern matching, and evaluation via `LeafExpressionConverter.EvaluateQuotation` (by @dbrattli)
 * [All] Add support for `Guid.CreateVersion7()` and `Guid.CreateVersion7(DateTimeOffset)` (by @OnurGumus)
 * [All] Add missing `Array`, `List`, and `Seq` random choice/shuffle/sample members and tests (by @ncave)
 * [Dart/Rust] Add missing `System.Random` implementations and tests (by @ncave)

src/Fable.Compiler/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+* [Python/Beam] Add F# quotation support — construction, pattern matching, and evaluation via `LeafExpressionConverter.EvaluateQuotation` (by @dbrattli)
 * [All] Add support for `Guid.CreateVersion7()` and `Guid.CreateVersion7(DateTimeOffset)` (by @OnurGumus)
 * [All] Add missing `Array`, `List`, and `Seq` random choice/shuffle/sample members and tests (by @ncave)
 * [Dart/Rust] Add missing `System.Random` implementations and tests (by @ncave)

src/Fable.Transforms/Beam/Fable2Beam.fs

@@ -1175,6 +1175,10 @@ let rec transformExpr (com: IBeamCompiler) (ctx: Context) (expr: Expr) : Beam.Er
                 ]
             )
 
+    | Quote(body, _isTyped, _r) ->
+        let emitted = QuotationEmitter.emitQuotedExpr com body
+        transformExpr com ctx emitted
+
     | Extended(kind, _range) ->
         match kind with
         | Throw(Some exprArg, _typ) ->

src/Fable.Transforms/Beam/Replacements.fs

@@ -5679,7 +5679,8 @@ let tryCall
             | _ -> emitExpr r t [ c ] "maps:get(name, $0)" |> Some
         | _ -> None
     | "System.Text.StringBuilder" -> bclType com ctx r t info thisArg args
-    | _ -> None
+    // F# Quotations
+    | typeName -> Quotations.tryQuotationCall "quotation" com ctx r t info thisArg args typeName
 
 let tryBaseConstructor
     (_com: ICompiler)

src/Fable.Transforms/Dart/Fable2Dart.fs

@@ -2191,6 +2191,10 @@ module Util =
             ],
             None
 
+        | Fable.Quote _ ->
+            addError com [] None "Quotations are not yet supported for Dart target"
+            [], None
+
     let getLocalFunctionGenericParams
         (_com: IDartCompiler)
         (ctx: Context)

src/Fable.Transforms/FSharp2Fable.fs

@@ -1456,10 +1456,11 @@ let private transformExpr (com: IFableCompiler) (ctx: Context) appliedGenArgs fs
                     $"Cannot compile ILFieldGet(%A{ownerTyp}, %s{fieldName})"
                     |> addErrorAndReturnNull com ctx.InlinePath (makeRangeFrom fsExpr)
 
-        | FSharpExprPatterns.Quote _ ->
-            return
-                "Quotes are not currently supported by Fable"
-                |> addErrorAndReturnNull com ctx.InlinePath (makeRangeFrom fsExpr)
+        | FSharpExprPatterns.Quote quotedExpr ->
+            let! body = transformExpr com ctx [] quotedExpr
+            let exprType = fsExpr.Type
+            let isTyped = exprType.GenericArguments.Count > 0
+            return Fable.Quote(body, isTyped, makeRangeFrom fsExpr)
 
         | FSharpExprPatterns.AddressOf expr ->
             let r = makeRangeFrom fsExpr
@@ -2475,6 +2476,8 @@ let resolveInlineExpr (com: IFableCompiler) ctx info expr =
             |> makeValue r
         | Fable.TypeInfo(t, d) -> Fable.TypeInfo(resolveInlineType ctx.GenericArgs t, d) |> makeValue r
 
+    | Fable.Quote(e, isTyped, r) -> Fable.Quote(resolveInlineExpr com ctx info e, isTyped, r)
+
     | Fable.Extended(kind, r) as e ->
         match kind with
         | Fable.Curry(e, arity) -> Fable.Extended(Fable.Curry(resolveInlineExpr com ctx info e, arity), r)

src/Fable.Transforms/Fable.Transforms.fsproj

@@ -28,6 +28,7 @@
     <Compile Include="Beam/Replacements.fs" />
     <Compile Include="Replacements.fs" />
     <Compile Include="Replacements.Api.fs" />
+    <Compile Include="QuotationEmitter.fs" />
     <Compile Include="FSharp2Fable.fsi" />
     <Compile Include="FSharp2Fable.fs" />
     <Compile Include="FableTransforms.fsi" />

src/Fable.Transforms/Fable2Babel.fs

@@ -1114,6 +1114,7 @@ module Util =
         | Fable.CurriedApply _
         | Fable.Operation _
         | Fable.Get _
+        | Fable.Quote _
         | Fable.Test _ -> false
 
         | Fable.TypeCast(e, _) -> isJsStatement ctx preferStatement e
@@ -3141,6 +3142,8 @@ but thanks to the optimisation done below we get
             | Fable.Throw _
             | Fable.Debugger -> iife com ctx expr
 
+        | Fable.Quote _ -> addErrorAndReturnNull com None "Quotations are not yet supported for JS/TS target"
+
     let rec transformAsStatements (com: IBabelCompiler) ctx returnStrategy (expr: Fable.Expr) : Statement array =
         match expr with
         | Fable.Unresolved(_, _, r) ->
@@ -3308,6 +3311,10 @@ but thanks to the optimisation done below we get
                 )
             |]
 
+        | Fable.Quote _ ->
+            addError com [] None "Quotations are not yet supported for JS/TS target"
+            [||]
+
     let transformFunction com ctx name (args: Fable.Ident list) (body: Fable.Expr) : Parameter array * BlockStatement =
         let tailcallChance =
             Option.map (fun name -> NamedTailCallOpportunity(com, ctx, name, args) :> ITailCallOpportunity) name

src/Fable.Transforms/FableTransforms.fs

@@ -159,7 +159,8 @@ let noSideEffectBeforeIdent identName expr =
             findIdentOrSideEffect e
         | Import _
         | Lambda _
-        | Delegate _ -> false
+        | Delegate _
+        | Quote _ -> false
         | Extended((Throw _ | Debugger), _) -> true
         | Extended(Curry(e, _), _) -> findIdentOrSideEffect e
         | CurriedApply(callee, args, _, _) -> callee :: args |> findIdentOrSideEffectInList |> orSideEffect