First impressions of Lean

Tue, 28 Apr 2026 06:49:00 +0000

Since reading a perfectable programming language I started getting the itch to try actual dependently typed programming languages again.

Haskell is my favorite programming language because it provides one of the strongest type system but my most common frustrations in Haskell are from not having access to stronger types. Dependent types in Haskell are an active research project, but still far away. So when I’ve wanted more power I’ve had to make do with Coq (now Roqc) plus a little bit of dabbling in Idris and Agda.

In my usual fashion, I dove right in without reading the tutorial, hoping my experience with similar languages would take me far enough.

First steps

I started out by just typing out some definitions.

def x : Int := 1
def y : Type := Int
def z : x := 1

I was surprised that the last example failed to compile:

▼ 2:14-2:15: error: failed to synthesize instance of type class OfNat x 1 numerals are polymorphic in Lean, but the numeral 1 cannot be used in a context where the expected type is x due to the absence of the instance above

This was fixable by using:

def y : x := (1 : Int)

I had a bit of trouble figuring out that Eq.refl needed an argument, but nicely _ allows inferring the type:

def x : Type := Int
def y : Int := 1
def z : x = Int := Eq.refl _

Of course, I later discovered that there’s a rfl tactic that allows avoiding this boilerplate.

I also ran into some other surprising sharp edges:

def decrement : Nat -> Nat
  | 0 => n
  | 1 + n => n

Doesn’t compile but this does:

def decrement : Nat -> Nat
  | 0 => n
  | n + 1 => n

It’s cool that it’s possible to pattern match on syntax like this, but it’s annoying that it’s a bit inconsistent.

Lambda Calculus Confluence

Basic syntax out of the way, I started implanting the simply typed lambda calculus.

Debrujin indexes:

inductive Term where
  | var : Nat -> Term
  | lam : Term -> Term
  | app : Term -> Term -> Term

Of course, I can’t implement a full interpreter because I can’t solve the halting problem, so I implement single step semantics:

def step_cbn (t : Term) : Maybe Term :=
  match t with
  | Term.lam _ => Maybe.nothing
  | Term.var _ => Maybe.nothing
  | Term.app (Term.lam body) t2 =>
      Maybe.just $ shift_down 0 (subst 0 (shift 1 0 t2) body)
  | Term.app t1 t2 => match step_cbn t1 with
    | Maybe.just t1' => Maybe.just $ Term.app t1' t2
    | Maybe.nothing => match step_cbn t2 with
      | Maybe.nothing => Maybe.nothing
      | Maybe.just t2' => Maybe.just $ Term.app t1 t2'

There’s several different evaluation orders for the simply typed lambda calculus. I recall that Church-Rosser theorem proves that when a term converges, it always converges to the same term regardless of evaluation order.

inductive ConvergesCBN : Term -> Term -> Type where
  | value : step_cbn v = Maybe.nothing -> ConvergesCBN v v
  | step  : step_cbn e = Maybe.just e' -> ConvergesCBN e' v -> ConvergesCBN e v

inductive ConvergesCBV : Term -> Term -> Type where
  | value : step_cbv v = Maybe.nothing -> ConvergesCBV v v
  | step : step_cbv e = Maybe.just e' -> ConvergesCBV e' v -> ConvergesCBV e v

noncomputable def cbv_implies_cbn_convergence :
  ConvergesCBV e v -> ConvergesCBN e v := sorry

I didn’t end up making that much progress. I also tried big step semantics, but that ended up not being much easier, and after Codex and Claude found several holes in my definitions, that made the theorem unprovable as stated, I gave up.

Ecosystem

Lean’s ecosystem still feels fairly immature.

parser libraries: I use megaparsec/attoparsec in Haskell
- fgdorais/lean4-parser: this provides decent parser combinators, but none of the fancier recovery or error handling offered by megaparsec.
C bindings are possible, but you’ll likely need to implement them yourself
effects: monad transformers are the default way of representing effects; usable, but a far cry from Haskell’s wide variety
tooling quite good, lean --server provides top good tooling even in Neovim. I appreciated this compared with using

A meetup

People writing Lean seem to be very dedicated to verifying things for verifications sake. I love that. I think this is the standard for what should be demanded of our libraries and tools.

I’m curious to try diving into metaprogramming, which I hear is pretty good, but that’ll have to wait for another day.

Unformed Delta - lean

First impressions of Lean

First steps

Lambda Calculus Confluence

Ecosystem

A meetup