From e13d73474181134fd08e14a0fbac2e947e52b44c Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Sun, 23 Aug 2026 18:06:00 +0200
Subject: [PATCH] feat(ast): add abstract syntax for expressions, patterns, and
 types

---
 src/Fml.Core/Ast.fs | 66 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)
 create mode 100644 src/Fml.Core/Ast.fs

diff --git a/src/Fml.Core/Ast.fs b/src/Fml.Core/Ast.fs
new file mode 100644
index 0000000..78ba49b
--- /dev/null
+++ b/src/Fml.Core/Ast.fs
@@ -0,0 +1,66 @@
+module Fml.Core.Ast
+
+// Abstract syntax for the fml language. Multi-argument functions are
+// desugared into nested single-argument lambdas by the parser, so the
+// evaluator and type checker only ever see one parameter per lambda.
+
+type Expr =
+    | IntLit of int
+    | BoolLit of bool
+    | StringLit of string
+    | UnitLit
+    | Var of string
+    | Lambda of string * Expr
+    | App of Expr * Expr
+    | Let of bool * string * Expr * Expr
+    | LetGroup of bool * (string * Expr) list * Expr
+    | If of Expr * Expr * Expr
+    | BinOp of string * Expr * Expr
+    | Neg of Expr
+    | Tuple of Expr list
+    | Nil
+    | Cons of Expr * Expr
+    | Match of Expr * (Pattern * Expr) list
+    | Construct of string * Expr list
+    | Record of (string * Expr) list
+    | Field of Expr * string
+
+and Pattern =
+    | PVar of string
+    | PWildcard
+    | PInt of int
+    | PBool of bool
+    | PString of string
+    | PUnit
+    | PTuple of Pattern list
+    | PNil
+    | PCons of Pattern * Pattern
+    | PConstruct of string * Pattern list
+    | PRecord of (string * Pattern) list
+
+// Surface syntax for type expressions inside declarations and annotations.
+type TyExpr =
+    | TEVar of string
+    | TECon of string * TyExpr list
+    | TETuple of TyExpr list
+    | TEArrow of TyExpr * TyExpr
+    | TEList of TyExpr
+
+type TypeBody =
+    | Variants of (string * TyExpr list) list
+    | RecordDef of (string * TyExpr) list
+
+type TypeDecl =
+    { Name: string
+      Params: string list
+      Body: TypeBody }
+
+// A top-level phrase is either a bare expression, a definition group, a type
+// declaration group, a module, or an open directive. Definitions extend the
+// persistent session environment.
+type Phrase =
+    | PhraseExpr of Expr
+    | PhraseLet of bool * (string * Expr) list
+    | PhraseType of TypeDecl list
+    | PhraseModule of string * Phrase list
+    | PhraseOpen of string