Lean-auto is an interface between Lean and automated theorem provers. Up to now, lean-auto is maintained and developed primarily by Yicheng Qian (GitHub: PratherConid). It is currently in active development, and pull requests/issues are welcome. For more information, feel free to reach out to Yicheng Qian on Lean Zulip.

Lean-auto is based on a monomorphization procedure from dependent type theory to higher-order logic and a deep embedding of higher-order logic into dependent type theory. It is capable of handling dependently-typed and/or universe-polymorphic input terms. Currently, proof reconstruction can be handled by Duper, a higher-order superposition prover written in Lean.

Although Lean-auto is still under development, it's already able to solve nontrivial problems. For example the first part of the "snake lemma" in category theory can be solved by a direct invocation to auto (and the second part can also be partly automated):

Type "auto 👍" to see whether auto is set up.

Usage

• auto [<term>,*] u[<ident>,*] d[<ident>,*]
  • u[<ident>,*]: Unfold identifiers
  • d[<ident>,*]: Add definitional equality related to identifiers
• Currently, auto supports
  • SMT solver invocation: set_option auto.smt true, but without proof reconstruction. Make sure that SMT solvers are installed, and that auto.smt.solver.name is correctly set.
  • TPTP Solver invocation: set_option auto.tptp true, but without proof reconstruction. Make sure that TPTP solvers (currently only supports zipperposition) are installed, and that auto.tptp.solver.name and auto.tptp.zeport.path are correctly set.
  • Proof search by native prover. To enable proof search by native prover, use set_option auto.native true, and use attribute [rebind Auto.Native.solverFunc] <solve_interface> to bind lean-auto to the interface of the solver, which should be a Lean constant of type Array Lemma → MetaM Expr.

Installing Lean-auto

• z3 version >= 4.12.2. Lower versions may not be able to deal with smt-lib 2.6 string escape sequence.
• cvc5
• zipperposition portfolio mode

Utilities

• Command #getExprAndApply [ <term> | <ident> ]: Defined in ExprExtra.lean. This command first elaborates the <term> into a lean Expr, then applies function <ident> to Expr. The constant ident must be already declared and be of type Expr → TermElabM Unit
• Command #genMonadState <term>, #genMonadContext <term>: Defined in MonadUtils.lean. Refer to the comment at the beginning of MonadUtils.lean.
• Command #fromMetaTactic [<ident>]: Calls Tactic.liftMetaTactic on <ident>. The constant <ident> must be already declared and be of type MVarId → MetaM (List MVarId)
• Lexical Analyzer Generator: Parser/LeanLex.lean. The frontend is not yet implemented. The backend can be found in NDFA.lean.

Monomorphization Strategy

• Let $H : \alpha$ be an assumption. We require that
  • $(1)$ If the type $\beta$ of any subterm $t$ of $\alpha$ depends on a bound variable $x$ inside $\alpha$, and $\beta$ is not of type $Prop$, then $x$ must be instantiated. Examples: Monomorphization, section InstExamples
  • $(2)$ If any binder $x$ of $\alpha$ has binderinfo instImplicit, then the binder $x$ must be instantiated via typeclass inference.
• TODO

Translation Workflow (Tentative)

• Collecting assumptions from local context and user-provided facts
  • We reduce let binders and unfold projections when we collect assumptions. So, in the following discussion, we'll assume that the expression contains no let binders and no projs.
  • We also $\beta$ reduce user provided facts so that there are nothing like $(\lambda x. t_1) \ t_2$
• $CIC \to COC$: Collecting constructors and recursors for inductive types (effectively, not directly)
  • e.g collecting equational theorem for match constructs
  • e.g collect constructors for inductively defined propositions
• $COC \to COC(\lambda^{c.u.})$: Monomorphization
  • Monomorphize all (dependently typed/universe polymorphic) facts to higher-order universe-monomorphic facts
  • $c.u.$ stands for "constant universe level"
  • Note that at this stage, all the facts we've obtained are still valid $CIC$ expressions and has convenient CIC proofs from the assumptions.
• $COC(\lambda^{c.u.}) \to COC(\lambda)$
  • We want all types $α$ occuring in the signature of constants and variables to be of sort Type (u + 1), i.e., $α : Type \ (u + 1)$. This is necessary because we want to write a checker (instead of directly reconstructing proof in DTT) and the valuation function from less expressive logic to dependent type theory requires [the elements in the range of the valuation function] to be [of the same sort].
  • To do this, we use GLift. For example, Nat.add is transformed into Nat.addLift

    structure GLift.{u, v} (α : Sort u) : Sort (max u (v + 1)) where
      /-- Lift a value into `GLift α` -/    up ::
      /-- Extract a value from `GLift α` -/ down : α
    
    def Nat.addLift.{u} (x y : GLift.{1, u} Nat) :=
      GLift.up (Nat.add (GLift.down x) (GLift.down y))

  • We only transfer these "lifted" terms to the less expressive $\lambda_2$, and $\lambda_2$ is unaware of the universe levels wrapped inside GLift.up.
  • Lifted constantes should be introduced into the local context. Theorems corresponding to the original one but using only lifted constants and with uniform universe levels, should also be introduced into the local context. Later translations should only use theorems and constants with uniform universe levels.
• $\lambda \to \lambda(\text{TPTP TF0})$: Instantiating function arguments
  • $\lambda$ is the reified $COC(\lambda)$
• There should also be a process similar to ULifting that "lifts" Bool into Prop, Nat to Int

Reification

• $COC(\lambda) \to \lambda(\text{TPTP\ TH0})$
  • Auto/Translation/LamReif.lean
• $\lambda(\text{TPTP TF0}) \to \text{TPTP TF0}$
  • Auto/Translation/LamFOL2SMT.lean

Checker

• The checker is based on a deep embedding of simply-typed lambda calculus into dependent type theory.
• The checker is slow on large input. For example, it takes 6s to typecheck the final example in BinderComplexity.lean. However, this is probably acceptable for mathlib usages, because e.g Mathlib/Analysis/BoxIntegral/DivergenceTheorem.lean has two theorems that take 4s to compile (but a large portion of the 4s are spent on typeclass inference)

Rules in Proof Tree

• defeq <num> <name>: The <num>-th definitional equality associated with definition <name>
• hw <name>: Lemmas hard-wired into Lean-auto
• lctxInh: Inhabitation fact from local context
• lctxLem: Lemma from local context
• queryNative::<func_name>: Proved by native prover
• rec <indName>.<ctorName>
• rw [0, 1]: Rewrite 0 using 1 (1 must be an equality)
• tyCanInh: Inhabitation instance synthesized for canonicalized type
• ❰<term>❱: User-provided lemma <term>