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README.md

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+# fml
+
+A small ML-family language implemented from scratch in F#.
+
+## Overview
+
+`fml` is a statically typed, functional mini-ML. It has full
+Hindley-Milner type inference (Algorithm W) with let-polymorphism,
+algebraic data types and records, exhaustive pattern matching, first-class
+functions and a small module system. Programs run on either of two
+back ends that share the same runtime values: a tree-walking interpreter
+and a bytecode compiler paired with a stack virtual machine. The test suite
+uses the two engines as an oracle for each other — every program must
+evaluate to the same value on both.
+
+## Features
+
+- **Lexer** — a hand-written tokenizer with nested block comments, string
+  escapes and type-variable syntax.
+- **Parser** — a recursive-descent parser with precedence climbing for
+  expressions, patterns, type declarations (variants and records), and
+  `struct ... end` modules with `open`.
+- **Type inference** — Hindley-Milner with unification over mutable type
+  cells, the level-based generalization trick (Rémy / OCaml), the occurs
+  check, and Algorithm W with full let-polymorphism.
+- **Algebraic data types and records** — user-declared parametric variants
+  and record types, checked and inferred throughout.
+- **Pattern matching** — normalization to a uniform constructor form,
+  Maranget's usefulness algorithm for exhaustiveness and redundancy
+  (with a rendered counterexample), and a decision-tree compiler.
+- **Two back ends** — a tree-walking interpreter and a stack-machine
+  compiler with a de Bruijn environment, closures, tail calls and
+  recursive knot-tying.
+- **Standard library** — `Option`, `Result`, `List` and `Map` modules
+  written in `fml` itself and loaded on demand.
+- **Tooling** — a REPL and a file runner.
+
+## Usage
+
+```sh
+# start the REPL
+dotnet run --project src/Fml.Cli
+
+# run a source file
+dotnet run --project src/Fml.Cli examples/demo.fml
+```
+
+In the REPL, enter an expression or a definition; each result is printed as
+`val <name> : <type> = <value>`. Press Ctrl-D to quit.
+
+```
+fml> let rec fact = fun n -> if n <= 1 then 1 else n * fact (n - 1) ;; fact 5
+val fact : int -> int = <fun>
+val it : int = 120
+```
+
+## Tests
+
+```sh
+dotnet test
+```
+
+The tests cover inference of polymorphic terms, type-error detection,
+evaluation, exhaustiveness and redundancy checking, the standard library,
+and an oracle that asserts the interpreter and the bytecode VM agree on a
+range of programs (recursion, higher-order functions, ADTs, records and
+modules).
+
+## Project structure
+
+```
+src/Fml.Core   the language: lexer, parser, types, inference, backends
+src/Fml.Cli    the REPL and file runner
+tests          xUnit test suite
+examples       sample fml programs
+```
+
+## Requirements
+
+- .NET 6 SDK (F#)