[JS/TS] Add F# quotation support with serializable QuotExpr AST

Add support for F# code quotations (<@ expr @> and <@@ expr @@>) targeting
JS/TS. Quotations compile to a serializable QuotExpr discriminated union
that works with Thoth.Json and Fable.Remoting for cross-platform
serialization between Fable clients and .NET backends.

Supported quotation nodes: values (int, float, string, char, bool, unit),
lambdas, let bindings, if/then/else, function calls, operators, tuples,
unions, records, options, lists, sequential, variable set, field get/set.

The % splice operator is supported for composing quotations.

New files:
- Fable.Core.Quotations.fs: shared F# DU types for .NET deserialization
- Quotation.ts: JS/TS runtime with proper Union subclasses
- QuotationEmitter.fs: compiler transform from Fable Quote AST to runtime calls
- QuotationTests.fs: test suite

Other targets (Python, Rust, Dart, Beam, PHP) have stubs with error messages.

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

Author: OnurGumus

src/Fable.AST/Fable.fs

@@ -834,6 +834,9 @@ type Expr =
     | TryCatch of body: Expr * catch: (Ident * Expr) option * finalizer: Expr option * range: SourceLocation option
     | IfThenElse of guardExpr: Expr * thenExpr: Expr * elseExpr: Expr * range: SourceLocation option
 
+    // Quotations
+    | Quote of quotedExpr: Expr * isTyped: bool
+
     | Unresolved of expr: UnresolvedExpr * typ: Type * range: SourceLocation option
     | Extended of expr: ExtendedSet * range: SourceLocation option
 
@@ -853,6 +856,7 @@ type Expr =
         | Get(_, _, t, _)
         | Emit(_, t, _)
         | DecisionTreeSuccess(_, _, t) -> t
+        | Quote _ -> Any
         | Set _
         | WhileLoop _
         | ForLoop _ -> Unit
@@ -874,7 +878,8 @@ type Expr =
         | Let _
         | LetRec _
         | DecisionTree _
-        | DecisionTreeSuccess _ -> None
+        | DecisionTreeSuccess _
+        | Quote _ -> None
         | Lambda(_, e, _)
         | Delegate(_, e, _, _)
         | TypeCast(e, _) -> e.Range

src/Fable.Cli/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Added
+
+* [JS/TS] Add F# quotation support with serializable `QuotExpr` AST (construction, pattern matching, JSON serialization)
+
 ## 5.0.0-rc.6 - 2026-03-31
 
 ### Fixed

src/Fable.Compiler/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Added
+
+* [JS/TS] Add F# quotation support with serializable `QuotExpr` AST (construction, pattern matching, JSON serialization)
+
 ## 5.0.0-rc.12 - 2026-03-31
 
 ### Fixed

src/Fable.Core/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Added
+
+* Add `Fable.Core.Quotations` module with `QuotExpr`, `QuotVar`, `QuotLiteral` types for cross-platform quotation serialization
+
 ## 5.0.0-rc.1 - 2026-02-26
 
 ### Added

src/Fable.Core/Fable.Core.Quotations.fs

@@ -0,0 +1,57 @@
+namespace Fable.Core.Quotations
+
+/// Serializable representation of literal values in quotations.
+/// All cases use primitive types for reliable cross-platform JSON serialization.
+[<RequireQualifiedAccess>]
+type QuotLiteral =
+    | Bool of bool
+    | SByte of sbyte
+    | Byte of byte
+    | Int16 of int16
+    | UInt16 of uint16
+    | Int of int
+    | UInt32 of uint32
+    | Int64 of int64
+    | UInt64 of uint64
+    | Float32 of float32
+    | Float of float
+    | Char of char
+    | String of string
+    | Decimal of decimal
+    | Unit
+    | Null of typeName: string
+
+/// Serializable representation of an F# quotation variable.
+type QuotVar =
+    {
+        Name: string
+        Type: string
+        IsMutable: bool
+    }
+
+/// Serializable representation of an F# quotation expression tree.
+/// Designed for cross-platform JSON serialization via Thoth.Json or Fable.Remoting.
+/// On Fable/JS: constructed by the compiler when you write <@ expr @>.
+/// On .NET: deserialize from JSON using the same type definitions.
+[<RequireQualifiedAccess>]
+type QuotExpr =
+    | Value of literal: QuotLiteral
+    | Var of var: QuotVar
+    | Lambda of var: QuotVar * body: QuotExpr
+    | Application of func: QuotExpr * arg: QuotExpr
+    | Let of var: QuotVar * value: QuotExpr * body: QuotExpr
+    | LetRecursive of bindings: (QuotVar * QuotExpr) array * body: QuotExpr
+    | IfThenElse of guard: QuotExpr * thenExpr: QuotExpr * elseExpr: QuotExpr
+    | Call of instance: QuotExpr option * methodName: string * declaringType: string * args: QuotExpr array
+    | Sequential of first: QuotExpr * second: QuotExpr
+    | NewTuple of elements: QuotExpr array
+    | TupleGet of tuple: QuotExpr * index: int
+    | NewUnionCase of typeName: string * tag: int * fields: QuotExpr array
+    | UnionCaseTag of expr: QuotExpr
+    | UnionCaseField of expr: QuotExpr * index: int
+    | NewRecord of typeName: string * fieldNames: string array * fields: QuotExpr array
+    | FieldGet of instance: QuotExpr option * fieldName: string
+    | FieldSet of instance: QuotExpr option * fieldName: string * value: QuotExpr
+    | VarSet of var: QuotVar * value: QuotExpr
+    | NewList of elements: QuotExpr array * elementType: string
+    | NewOption of value: QuotExpr option * elementType: string

src/Fable.Core/Fable.Core.fsproj

@@ -20,6 +20,7 @@
     <Compile Include="Fable.Core.PyInterop.fs" />
     <Compile Include="Fable.Core.RustInterop.fs" />
     <Compile Include="Fable.Core.BeamInterop.fs" />
+    <Compile Include="Fable.Core.Quotations.fs" />
     <Compile Include="Fable.Core.Extensions.fs" />
     <Content Include="CHANGELOG.md" />
   </ItemGroup>

src/Fable.Transforms/Beam/Fable2Beam.fs

@@ -1175,6 +1175,10 @@ let rec transformExpr (com: IBeamCompiler) (ctx: Context) (expr: Expr) : Beam.Er
                 ]
             )
 
+    | Quote _ ->
+        com.WarnOnlyOnce("Quotations are not supported for Beam target", ?range = expr.Range)
+        Beam.ErlExpr.Literal(Beam.ErlLiteral.AtomLit(Beam.Atom "undefined"))
+
     | Extended(kind, _range) ->
         match kind with
         | Throw(Some exprArg, _typ) ->

src/Fable.Transforms/Dart/Fable2Dart.fs

@@ -1986,6 +1986,10 @@ module Util =
             addError com [] r "Unexpected unresolved expression"
             [], None
 
+        | Fable.Quote _ ->
+            addError com [] expr.Range "Quotations are not supported for Dart"
+            [], None
+
         | Fable.ObjectExpr(_, t, _) ->
             match returnStrategy with
             // Constructors usually have a useless object expression on top

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)
 
         | FSharpExprPatterns.AddressOf expr ->
             let r = makeRangeFrom fsExpr
@@ -2485,6 +2486,8 @@ let resolveInlineExpr (com: IFableCompiler) ctx info expr =
             )
         | Fable.Debugger -> e
 
+    | Fable.Quote(quotedExpr, isTyped) -> Fable.Quote(resolveInlineExpr com ctx info quotedExpr, isTyped)
+
     | Fable.Unresolved(e, t, r) ->
         match e with
         | Fable.UnresolvedTraitCall(sourceTypes, traitName, isInstance, argTypes, argExprs) ->

src/Fable.Transforms/Fable.Transforms.fsproj

@@ -27,6 +27,7 @@
     <Compile Include="Beam/Beam.AST.fs" />
     <Compile Include="Beam/Replacements.fs" />
     <Compile Include="Replacements.fs" />
+    <Compile Include="QuotationEmitter.fs" />
     <Compile Include="Replacements.Api.fs" />
     <Compile Include="FSharp2Fable.fsi" />
     <Compile Include="FSharp2Fable.fs" />