README.md

compiler.lean

Inspired by the wonderful Program Correctness video over on Computerphile, this repo contains a formally verified "compiler" for a language with natural number values and add expressions. It is written using the Lean theorem prover.

We define an expression type for our language and an instruction type for the stack machine which our compiler targets.

inductive Expr
| Val : ℕ -> Expr
| Add : Expr -> Expr -> Expr

inductive Instr
| PUSH : ℕ -> Instr
| ADD : Instr

exec_compile_eq_eval (e : Expr) : exec (compile e) [] = [eval e]

[Source]

All expressions e when compiled and executed on an empty stack produce the same value as eval.

I'm still new to lean so my proofs aren't great. Suggestions welcome!

eval : Expr -> ℕ

Evaluates an Expr to produce a natural number

eval (Val 5)
=> 5
eval (Add (Add (Val 10) (Val 20))
          (Val 30))
=> 60

compile : Expr -> list Instr

Compiles an Expr to produce a list of instructions

compile (Val 42)
=> [PUSH 42]
compile (Add (Add (Val 10) (Val 20))
             (Val 30))
=> [PUSH 10, PUSH 20, ADD, PUSH 30, ADD]

exec : list Instr -> list ℕ -> list ℕ

Executes a list of instructions on a stack

exec [PUSH 42] []
=> [42]
exec [PUSH 10, PUSH 20, ADD, PUSH 30, ADD][]
=> [60]