Aesop (Automated Extensible Search for Obvious Proofs) is a proof search tactic for Lean 4. It is broadly similar to Isabelle's auto. In essence, Aesop works like this:

  • As with simp, you tag a (large) collection of definitions with the @[aesop] attribute, registering them as Aesop rules. Rules can be arbitrary tactics. We provide convenient ways to create common types of rules, e.g. rules which apply a lemma.
  • Aesop takes these rules and tries to apply each of them to the initial goal. If a rule succeeds and generates subgoals, Aesop recursively applies the rules to these subgoals, building a search tree.
  • The search tree is explored in a best-first manner. You can mark rules as more or less likely to be useful. Based on this information, Aesop prioritises the goals in the search tree, visiting more promising goals before less promising ones.
  • Before any rules are applied to a goal, it is normalised, using a special (customisable) set of normalisation rules. An important built-in normalisation rule runs simp_all, so your @[simp] lemmas are taken into account by Aesop.
  • Rules can be marked as safe to optimise Aesop's performance. A safe rule is applied eagerly and is never backtracked. For example, Aesop's built-in rules safely split a goal P ∧ Q into goals for P and Q. After this split, the original goal P ∧ Q is never revisited.
  • Aesop provides a set of built-in rules which perform logical operations (e.g. case-split on hypotheses P ∨ Q) and some other straightforward deductions.
  • Aesop uses indexing methods similar to those of simp and other Lean tactics. This means it should remain reasonably fast even with a large rule set.
  • When called as aesop?, Aesop prints a tactic script that proves the goal, similar to simp?. This way you can avoid the performance penalty of running Aesop all the time. However, the script generation is currently not fully reliable, so you may have to adjust the generated script.

Aesop is suitable for two main use cases:

  • General-purpose automation, where Aesop is used to dispatch 'trivial' goals. By registering enough lemmas as Aesop rules, you can turn Aesop into a much more powerful simp.
  • Special-purpose automation, where specific Aesop rule sets are built to address a certain class of goals. Mathlib tactics such as measurability and continuity are implemented by Aesop.

I only occasionally update this README, so details may be out of date. If you have questions, please create an issue or ping me (Jannis Limperg) on the Lean Zulip. Pull requests are very welcome!

There's also a paper about Aesop which covers many of the topics discussed here, sometimes in more detail.


With elan installed, lake build should suffice.

Adding Aesop to Your Project

To use Aesop in a Lean 4 project, first add this package as a dependency. In your lakefile.lean, add

require aesop from git ""

You also need to make sure that your lean-toolchain file contains the same version of Lean 4 as Aesop's, and that your versions of Aesop's dependencies (currently only std4) match. We unfortunately can't support version ranges at the moment.

Now the following test file should compile:

import Aesop

example : α → α :=
  by aesop


To get you started, I'll explain Aesop's major concepts with a series of examples. A more thorough, reference-style discussion follows in the next section.

We first define our own version of lists (so as not to clash with the standard library) and an append function:

inductive MyList (α : Type _)
  | nil
  | cons (hd : α) (tl : MyList α)

namespace MyList

protected def append : (_ _ : MyList α) → MyList α
  | nil, ys => ys
  | cons x xs, ys => cons x (MyList.append xs ys)

instance : Append (MyList α) :=

We also tell simp to unfold applications of append:

theorem nil_append : nil ++ xs = xs := rfl

theorem cons_append : cons x xs ++ ys = cons x (xs ++ ys) := rfl

When Aesop first encounters a goal, it normalises it by running a customisable set of normalisation rules. One such normalisation rule effectively runs simp_all, so Aesop automatically takes simp lemmas into account.

Now we define the NonEmpty predicate on MyList:

@[aesop safe [constructors, cases]]
inductive NonEmpty : MyList α → Prop
  | cons : NonEmpty (cons x xs)

Here we see the first proper Aesop feature: we use the @[aesop] attribute to construct two Aesop rules related to the NonEmpty type. These rules are added to a global rule set. When Aesop searches for a proof, it systematically applies each available rule, then recursively searches for proofs of the subgoals generated by the rule, and so on, building a search tree. A goal is proved when Aesop applies a rule that generates no subgoals.

In general, rules can be arbitrary tactics. But since you probably don't want to write a tactic for every rule, the aesop attribute provides several rule builders which construct common sorts of rules. In our example, we construct:

  • A constructors rule. This rule tries to apply each constructor of NonEmpty whenever a goal has target NonEmpty _.
  • A cases rule. This rule searches for hypotheses h : NonEmpty _ and performs case analysis on them (like the cases tactic).

Both rules above are added as safe rules. When a safe rule succeeds on a goal encountered during the proof search, it is applied and the goal is never visited again. In other words, the search does not backtrack safe rules. We will later see unsafe rules, which can backtrack.

With these rules, we can prove a theorem about NonEmpty and append:

@[aesop unsafe 50% apply]
theorem nonEmpty_append₁ {xs : MyList α} ys :
    NonEmpty xs → NonEmpty (xs ++ ys) := by

Aesop finds this proof in four steps:

  • A built-in rule introduces the hypothesis h : NonEmpty xs. By default, Aesop's rule set contains a number of straightforward rules for handling the logical connectives , , and ¬ as well as the quantifiers and and some other basic types.
  • The cases rule for NonEmpty performs case analysis on h.
  • The simp rule cons_append, which we added earlier, unfolds the ++ operation.
  • The constructor rule for NonEmpty applies NonEmpty.cons.

If you want to see how Aesop proves your goal (or why it doesn't prove your goal, or why it takes too long to prove your goal), you can enable tracing:

set_option trace.aesop true

This makes Aesop print out the steps it takes while searching for a proof. You can also look at the search tree Aesop constructed by enabling the trace.aesop.tree option. For more tracing options, type set_option trace.aesop and see what auto-completion suggests.

If, in the example above, you call aesop? instead, then Aesop prints a proof script. At time of writing, it looks like this:

intro a
obtain @⟨x, xs_1⟩ := a
simp_all only [cons_append]
apply MyList.NonEmpty.cons

With a bit of post-processing, you can use this script instead of the Aesop call. This way you avoid the performance penalty of making Aesop search for a proof over and over again. The proof script generation currently has some known bugs, but it produces usable scripts most of the time.

The @[aesop] attribute on nonEmpty_append₁ adds this lemma as an unsafe rule to the default rule set. For this rule we use the apply rule builder, which generates a rule that tries to apply nonEmpty_append₁ whenever the target is of the form NonEmpty (_ ++ _).

Unsafe rules are rules which can backtrack, so after they have been applied to a goal, Aesop may still try other rules to solve the same goal. This makes sense for nonEmpty_append₁: if we have a goal NonEmpty (xs ++ ys), we may prove it either by showing NonEmpty xs (i.e., by applying nonEmpty_append₁) or by showing NonEmpty ys. If nonEmpty_append₁ were registered as a safe rule, we would always choose NonEmpty xs and never investigate NonEmpty ys.

Each unsafe rule is annotated with a success probability, here 50%. This is a very rough estimate of how likely it is that the rule will to lead to a successful proof. It is used to prioritise goals: the initial goal starts with a priority of 100% and whenever we apply an unsafe rule, the priority of its subgoals is the priority of its parent goal multiplied with the success probability of the applied rule. So applying nonEmpty_append₁ repeatedly would give us goals with priority 50%, 25%, etc. Aesop always considers the highest-priority unsolved goal first, so it prefers proof attempts involving few and high-probability rules. Additionally, when Aesop has a choice between multiple unsafe rules, it prefers the one with the highest success probability. (Ties are broken arbitrarily but deterministically.)

After adding nonEmpty_append, Aesop can prove some consequences of this lemma:

example {α : Type u} {xs : MyList α} ys zs :
    NonEmpty xs → NonEmpty (xs ++ ys ++ zs) := by

Next, we prove another simple theorem about NonEmpty:

theorem nil_not_nonEmpty (xs : MyList α) : xs = nil → ¬ NonEmpty xs := by
  aesop (add 10% cases MyList)

Here we use an add clause to add a rule which is only used in this specific Aesop call. The rule is an unsafe cases rule for MyList. (As you can see, you can leave out the unsafe keyword and specify only a success probability.) This rule is dangerous: when we apply it to a hypothesis xs : MyList α, we get x : α and ys : MyList α, so we can apply the cases rule again to ys, and so on. We therefore give this rule a very low success probability, to make sure that Aesop applies other rules if possible.

Here are some examples where Aesop's normalisation phase is particularly useful:

theorem append_nil {xs : MyList α} :
    xs ++ nil = xs := by
  induction xs <;> aesop

theorem append_assoc {xs ys zs : MyList α} :
    (xs ++ ys) ++ zs = xs ++ (ys ++ zs) := by
  induction xs <;> aesop

Since we previously added unfolding lemmas for append to the global simp set, Aesop can prove theorems about this function more or less by itself (though in fact simp_all would already suffice.) However, we still need to perform induction explicitly. This is a deliberate design choice: techniques for automating induction exist, but they are complex, somewhat slow and not entirely reliable, so we prefer to do it manually.

Many more examples can be found in the AesopTest folder of this repository. In particular, the file AesopTest/List.lean contains an Aesop-ified port of 200 basic list lemmas from the Lean 3 version of mathlib, and the file AesopTest/SeqCalcProver.lean shows how Aesop can help with the formalisation of a simple sequent calculus prover.


This section contains a systematic and fairly comprehensive account of how Aesop operates.


A rule is a tactic plus some associated metadata. Rules come in three flavours:

  • Normalisation rules (keyword norm) must generate zero or one subgoal. (Zero means that the rule closed the goal). Each normalisation rule is associated with an integer penalty (default 1). Normalisation rules are applied in a fixpoint loop in order of penalty, lowest first. For rules with equal penalties, the order is unspecified. See below for details on the normalisation algorithm.

    Normalisation rules can also be simp lemmas. These are constructed with the simp builder. They are used by a special simp call during the normalisation process.

  • Safe rules (keyword safe) are applied after normalisation but before any unsafe rules. When a safe rule is successfully applied to a goal, the goal becomes inactive, meaning no other rules are considered for it. Like normalisation rules, safe rules are associated with a penalty (default 1) which determines the order in which the rules are tried.

    Safe rules should be provability-preserving, meaning that if a goal is provable and we apply a safe rule to it, the generated subgoals should still be provable. This is a less precise notion than it may appear since what is provable depends on the entire Aesop rule set.

  • Unsafe rules (keyword unsafe) are tried only if all available safe rules have failed on a goal. When an unsafe rule is applied to a goal, the goal is not marked as inactive, so other (unsafe) rules may be applied to it. These rule applications are considered independently until one of them proves the goal (or until we've exhausted all available rules and determine that the goal is not provable with the current rule set).

    Each unsafe rule has a success probability between 0% and 100%. These probabilities are used to determine the priority of a goal. The initial goal has priority 100% and whenever we apply an unsafe rule, the priorities of its subgoals are the priority of the rule's parent goal times the rule's success probability. Safe rules are treated as having 100% success probability.

Rules can also be multi-rules. These are rules which add multiple rule applications to a goal. For example, registering the constructors of the Or type will generate a multi-rule that, given a goal with target A ∨ B, generates one rule application with goal A and one with goal B. This is equivalent to registering one rule for each constructor, but multi-rules can be both slightly more efficient and slightly more natural.

Search Tree

Aesop's central data structure is a search tree. This tree alternates between two kinds of nodes:

  • Goal nodes: these nodes store a goal, plus metadata relevant to the search. The parent and children of a goal node are rule application nodes. In particular, each goal node has a priority between 0% and 100%.
  • Rule application ('rapp') nodes: these goals store a rule (plus metadata). The parent and children of a rapp node are goal nodes. When the search tree contains a rapp node with rule r, parent p and children c₁, ..., cₙ, this means that the tactic of rule r was applied to the goal of p, generating the subgoals of the cᵢ.

When a goal node has multiple child rapp nodes, we have a choice of how to solve the goals. This makes the tree an AND-OR tree: to prove a rapp, all its child goals must be proved; to prove a goal, any of its child rapps must be proved.


We start with a search tree containing a single goal node. This node's goal is the goal which Aesop is supposed to solve. Then we perform the following steps in a loop, stopping if (a) the root goal has been proved; (b) the root goal becomes unprovable; or (c) one of Aesop's rule limits has been reached. (There are configurable limits on, e.g., the total number of rules applied or the search depth.)

  • Pick the highest-priority active goal node G. Roughly speaking, a goal node is active if it is not proved and we haven't yet applied all possible rules to it.

  • If the goal of G has not been normalised yet, normalise it. That means we run the following normalisation loop:

    • Run the normalisation rules with negative penalty (lowest penalty first). If any of these rules is successful, restart the normalisation loop with the goal produced by the rule.
    • Run simp on all hypotheses and the target, using the global simp set (i.e. lemmas tagged @[simp]) plus Aesop's simp rules.
    • Run the normalisation rules with positive penalty (lowest penalty first). If any of these rules is successful, restart the normalisation loop.

    The loop ends when all normalisation rules fail. It destructively updates the goal of G (and may prove it outright).

  • If we haven't tried to apply the safe rules to the goal of G yet, try to apply each safe rule (lowest penalty first). As soon as a rule succeeds, add the corresponding rapp and child goals to the tree and mark G as inactive. The child goals receive the same priority as G.

  • Otherwise there is at least one unsafe rule that hasn't been tried on G yet (or else G would have been inactive). Try the unsafe rule with the highest success probability and if it succeeds, add the corresponding rapp and child goals to the tree. The child goals receive the priority of G times the success probability of the applied rule.

A goal is unprovable if we have applied all possible rules to it and all resulting child rapps are unprovable. A rapp is unprovable if any of its subgoals is unprovable.

During the search, a goal or rapp can also become irrelevant. This means that we don't have to visit it again. Informally, goals and rapps are irrelevant if they are part of a branch of the search tree which has either successfully proved its goal already or which can never prove its goal. More formally:

  • A goal is irrelevant if its parent rapp is unprovable. (This means that a sibling of the goal is already unprovable, in which case we know that the parent rapp will never be proved.)
  • A rapp is irrelevant if its parent goal is proved. (This means that a sibling of the rapp is already proved, and we only need one proof.)
  • A goal or rapp is irrelevant if any of its ancestors is irrelevant.

Rule Builders

A rule builder is a metaprogram that turns an expression into an Aesop rule. When you tag a declaration with the @[aesop] attribute, the builder is applied to the declared constant. When you use the add clause, as in (add <phase> <builder> (<term>)), the builder is applied to the given term, which may involve hypotheses from the goal. However, some builders only support global constants. If the term is a single identifier, e.g. the name of a hypothesis, the parentheses around it are optional.

Currently available builders are:

  • apply: generates a rule which acts like the apply tactic.

  • forward: when applied to a term of type A₁ → ... Aₙ → B, generates a rule which looks for hypotheses h₁ : A₁, ..., hₙ : Aₙ in the goal and, if they are found, adds a new hypothesis h : B. As an example, consider the lemma even_or_odd:

    even_or_odd : ∀ (n : Nat), Even n ∨ Odd n

    Registering this as a forward rule will cause the goal

    n : Nat
    m : Nat
    ⊢ T

    to be transformed into this:

    n : Nat
    hn : Even n ∨ Odd n
    m : Nat
    hm : Even m ∨ Odd m
    ⊢ T

    The forward builder may also be given a list of immediate names:

    forward (immediate := [n]) even_or_odd

    The immediate names, here n, refer to the arguments of even_or_odd. When Aesop applies a forward rule with explicit immediate names, it only matches the corresponding arguments to hypotheses. (Here, even_or_odd has only one argument, so there is no difference.)

    When no immediate names are given, Aesop considers every argument immediate, except for instance arguments and dependent arguments (i.e. those that can be inferred from the types of later arguments).

    When a forward rule is successful, Aesop remembers the type of the hypothesis added by the rule, say T. If a forward rule (possibly the same one) is subsequently applied to a subgoal and wants to add another hypothesis of type T, this is forbidden and the rule fails. Without this restriction, forward rules would in many cases be applied infinitely often. However, note that the rule is still executed on its own subgoals (and their subgoals, etc.), which can become a performance issue. You should therefore prefer destruct rules where possible.

  • destruct: works like forward, but after the rule has been applied, hypotheses that were used as immediate arguments are cleared. This is useful when you want to eliminate a hypothesis. E.g. the rule

    @[aesop norm destruct]
    theorem and_elim_right : α ∧ β → α := ...

    will cause the goal

    h₁ : (α ∧ β) ∧ γ
    h₂ : δ ∧ ε

    to be transformed into

    h₁ : α
    h₂ : δ

    Unlike with forward rules, when an destruct rule is successfully applied, it may be applied again to the resulting subgoals (and their subgoals, etc.). There is less danger of infinite cycles because the original hypothesis is cleared.

    However, if the hypothesis or hypotheses to which the destruct rule is applied have dependencies, they are not cleared. In this case, you'll probably get an infinite cycle.

  • constructors: when applied to an inductive type or structure T, generates a rule which tries to apply each constructor of T to the target. This is a multi-rule, so if multiple constructors apply, they are considered in parallel. If you use this constructor to build an unsafe rule, each constructor application receives the same success probability; if this is not what you want, add separate apply rules for the constructors.

  • cases: when applied to an inductive type or structure T, generates a rule that performs case analysis on every hypothesis h : T in the context. The rule recurses into subgoals, so cases Or will generate 6 goals when applied to a goal with hypotheses h₁ : A ∨ B ∨ C and h₂ : D ∨ E. However, if T is a recursive type (e.g. List), we only perform case analysis once on each hypothesis. Otherwise we would loop infinitely.

    The cases_patterns option can be used to apply the rule only on hypotheses of a certain shape. E.g. the rule cases (cases_patterns := [Fin 0]) Fin will perform case analysis only on hypotheses of type Fin 0. Patterns can contain underscores, e.g. 0 ≤ _. Multiple patterns can be given (separated by commas); the rule is then applied whenever at least one of the patterns matches a hypothesis.

  • simp: when applied to an equation eq : A₁ → ... Aₙ → x = y, registers eq as a simp lemma for the built-in simp pass during normalisation. As such, this builder can only build normalisation rules.

  • unfold: when applied to a definition or let hypothesis f, registers f to be unfolded (i.e. replaced with its definition) during normalisation. As such, this builder can only build normalisation rules. The unfolding happens in a separate simp pass.

    The simp builder can also be used to unfold definitions. The difference is that simp rules perform smart unfolding (like the simp tactic) and unfold rules perform non-smart unfolding (like the unfold tactic). Non-smart unfolding unfolds functions even when none of their equations match, so unfold rules would lead to looping and are forbidden.

  • tactic: takes a tactic and directly turns it into a rule. When this builder is used in an add clause, you can use e.g. (add safe (by norm_num)) to register norm_num as a safe rule. The by block can also contain multiple tactics as well as references to the hypotheses. When you use (by ...) in an add clause, Aesop automatically uses the tactic builder, unless you specify a different builder.

    When this builder is used in the @[aesop] attribute, the declaration tagged with the attribute must have type TacticM Unit, Aesop.SingleRuleTac or Aesop.RuleTac. The latter are Aesop data types which associate a tactic with additional metadata; using them may allow the rule to operate somewhat more efficiently.

    Rule tactics should not be 'no-ops': if a rule tactic is not applicable to a goal, it should fail rather than return the goal unchanged. All no-op rules waste time; no-op norm rules will send normalisation into an infinite loop; and no-op safe rules will prevent unsafe rules from being applied.

    Normalisation rules may not assign metavariables (other than the goal metavariable) or introduce new metavariables (other than the new goal metavariable). This can be a problem because some Lean tactics, e.g. cases, do so even in situations where you probably would not expect them to. I'm afraid there is currently no good solution for this.

  • default: The default builder. This is the builder used when you register a rule without specifying a builder, but you can also use it explicitly. Depending on the rule's phase, the default builder tries different builders, using the first one that works. These builders are:

    • For safe and unsafe rules: constructors, tactic, apply.
    • For norm rules: constructors, tactic, simp, apply.
Transparency Options

The rule builders apply, forward, destruct, constructors and cases each have a transparency option. This option controls the transparency at which the rule is executed. For example, registering a rule with the builder apply (transparency := reducible) makes the rule act like the tactic with_reducible apply.

However, even if you change the transparency of a rule, it is still indexed at reducible transparency (since the data structure we use for indexing only supports reducible transparency). So suppose you register an apply rule with default transparency. Further suppose the rule concludes A ∧ B and your target is T with def T := A ∧ B. Then the rule could apply to the target since it can unfold T at default transparency to discover A ∧ B. However, the rule is never applied because the indexing procedure sees only T and does not consider the rule potentially applicable.

To override this behaviour, you can write apply (transparency! := default) (note the bang). This disables indexing, so the rule is tried on every goal.

Rule Sets

Rule sets are declared with the command

declare_aesop_rule_sets [r₁, ..., rₙ] (default := <bool>)

where the rᵢ are arbitrary names. To avoid clashes, pick names in the namespace of your package. Setting default := true makes the rule set active by default. The default clause can be omitted and defaults to false.

Within a rule set, rules are identified by their name, builder and phase (safe/unsafe/norm). This means you can add the same declaration as multiple rules with different builders or in different phases, but not with different priorities or different builder options (if the rule's builder has any options).

Rules can appear in multiple rule sets, but in this case you should make sure that they have the same priority and use the same builder options. Otherwise, Aesop will consider these rules the same and arbitrarily pick one.

Out of the box, Aesop uses the default rule sets builtin and default. The builtin set contains built-in rules for handling various constructions (see below). The default set contains rules which were added by Aesop users without specifying a rule set.

The @[aesop] Attribute

Declarations can be added to rule sets by annotating them with the @[aesop] attribute. As with other attributes, you can use @[local aesop] to add a rule only within the current section or namespace and @[scoped aesop] to add a rule only when the current namespace is open.

Single Rule

In most cases, you'll want to add one rule for the declaration. The syntax for this is

@[aesop <phase>? <priority>? <builder>? <builder_option>* <rule_sets>?]


  • <phase> is safe, norm or unsafe. Cannot be omitted except under the conditions in the next bullets.

  • <priority> is:

    • For simp rules, a natural number. This is used as the priority of the simp generated simp lemmas, so registering a simp rule with priority n is roughly equivalent to the attribute @[simp n]. If omitted, defaults to Lean's default simp priority.
    • For safe and norm rules (except simp rules), an integer penalty. If omitted, defaults to 1.
    • For unsafe rules, a percentage between 0% and 100%. Cannot be omitted. You may omit the unsafe phase specification when giving a percentage.
    • For unfold rules, a penalty can be given, but it is currently ignored.
  • <builder> is one of the builders given above. If no builder is specified, the default builder for the given phase is used.

    When the simp builder is used, the norm phase may be omitted since this builder can only generate normalisation rules.

  • <builder_option>* is a list of zero or more builder options. See above for the different builders' options.

  • <rule_sets> is a clause of the form

    (rule_sets := [r₁, ..., rₙ])

    where the rᵢ are declared rule sets. (Parentheses are mandatory.) The rule is added exactly to the specified rule sets. If this clause is omitted, it defaults to (rule_sets := [default]).

Multiple Rules

It is occasionally useful to add multiple rules for a single declaration, e.g. a cases and a constructors rule for the same inductive type. In this case, you can write for example

@[aesop unsafe [constructors 75%, cases 90%]]
inductive T ...

@[aesop apply [safe (rule_sets := [A]), 70% (rule_sets := [B])]]
def foo ...

@[aesop [80% apply, safe 5 forward (immediate := x)]]
def bar (x : T) ...

In the first example, two unsafe rules for T are registered, one with success probability 75% and one with 90%.

In the second example, two rules are registered for foo. Both use the apply builder. The first, a safe rule with default penalty, is added to rule set A. The second, an unsafe rule with 70% success probability, is added to rule set B.

In the third example, two rules are registered for bar: an unsafe rule with 80% success probability using the apply builder and a safe rule with penalty 5 using the forward builder.

In general, the grammar for the @[aesop] attribute is

attr      ::= @[aesop <rule_expr>]
            | @[aesop [<rule_expr,+>]]

rule_expr ::= feature
            | feature <rule_expr>
            | feature [<rule_expr,+>]

where feature is a phase, priority, builder or rule_sets clause. This grammar yields one or more trees of features and each branch of these trees specifies one rule. (A branch is a list of features.)

Adding External Rules

You can use the attribute command to add rules for constants which were declared previously, either in your own development or in a package you import:

attribute [aesop norm unfold] List.all -- List.all is from Init

You can also use the add_aesop_rules command:

add_aesop_rules safe [(by linarith), Nat.add_comm 0]

As you can see, this command can be used to add tactics and composite terms as well. Use local add_aesop_rules and scoped add_aesop_rules to obtain the equivalent of @[local aesop] and @[scoped aesop].

Erasing Rules

There are two ways to erase rules. Usually it suffices to remove the @[aesop] attribute:

attribute [-aesop] foo

This will remove all rules associated with the declaration foo from all rule sets. However, this erasing is not persistent, so the rule will reappear at the end of the file. This is a fundamental limitation of Lean's attribute system: once a declaration is tagged with an attribute, it cannot be permanently untagged.

If you want to remove only certain rules, you can use the erase_aesop_rules command:

erase_aesop_rules [safe apply foo, bar (rule_sets := [A])]

This will remove:

  • all safe rules for foo with the apply builder from all rule sets (but not other, for example, unsafe rules or forward rules);
  • all rules for bar from rule set A.

In general, the syntax is

erase_aesop_rules [<rule_expr,+>]

i.e. rules are specified in the same way as for the @[aesop] attribute. However, each rule must also specify the name of the declaration whose rules should be erased. The rule_expr grammar is therefore extended such that a feature can also be the name of a declaration.

Note that a rule added with one of the default builders (safe_default, norm_default, unsafe_default) will be registered under the name of the builder that is ultimately used, e.g. apply or simp. So if you want to erase such a rule, you may have to specify that builder instead of the default builder.

The aesop Tactic

In its most basic form, you can call the Aesop tactic just by writing

example : α → α := by

This will use the rules in the default rule sets. Out of the box, these are the default rule set, containing rules tagged with the @[aesop] attribute without mentioning a specific rule set, and the builtin rule set, containing rules built into Aesop. However, other rule sets can also be enabled by default; see the declare_aesop_rule_sets command.

The tactic's behaviour can also be customised with various options. A more involved Aesop call might look like this:

  (add safe foo, 10% cases Or, safe cases Empty)
  (erase A, baz)
  (rule_sets := [A, B])
  (config := { maxRuleApplicationDepth := 10 })

Here we add some rules with an add clause, erase other rules with an erase clause, limit the used rule sets and set some options. Each of these clauses is discussed in more detail below.

Adding Rules to an Aesop Call

Rules can be added to an Aesop call with an add clause. This won't affect any declared rule sets. The syntax of the add clause is

(add <rule_expr,+>)

i.e. rules can be specified in the same way as for the @[aesop] attribute. As with the erase_aesop_rules command, each rule must specify the name of declaration from which the rule should be built; for example

(add safe [foo 1, bar 5])

will add the declaration foo as a safe rule with penalty 1 and bar as a safe rule with penalty 5.

The rule names can also refer to hypotheses in the goal context, but not all builders support this.

Erasing Rules From an Aesop Call

Rules can be removed from an Aesop call with an erase clause. Again, this affects only the current Aesop call and not the declared rule sets. The syntax of the erase clause is

(erase <rule_expr,+>)

and it works exactly like the erase_aesop_rules command. To erase all rules associated with x and y, write

(erase x, y)
Selecting Rule Sets

By default, Aesop uses the default and builtin rule sets, as well as rule sets which are declared as default rule sets. A rule_sets clause can be given to include additional rule sets, e.g.

(rule_sets := [A, B])

This will use rule sets A, B, default and builtin (and any rule sets declared as default rule sets). Rule sets can also be disabled with rule_sets := [-default, -builtin].

Setting Options

Various options can be set with a config clause, whose syntax is:

(config := <term>)

The term is an arbitrary Lean expression of type Aesop.Options; see there for details. Notable options include:

  • strategy selects a best-first, depth-first or breadth-first search strategy. The default is best-first.
  • useSimpAll := false makes the built-in simp rule use simp at * rather than simp_all.
  • enableSimp := false disables the built-in simp rule altogether.

Similarly, options for the built-in norm simp rule can be set with

(simp_config := <term>)

You can give the same options here as in simp (config := ...).

Built-In Rules

The set of built-in rules (those in the builtin rule set) is a bit unstable, so for now I won't document them in detail. See Aesop/BuiltinRules.lean and Aesop/BuiltinRules/*.lean

Proof Scripts

By calling aesop? instead of aesop, you can instruct Aesop to generate a tactic script which proves the goal (if Aesop succeeds). The script is printed as a Try this: suggestion, similar to simp?.

The scripts generated by Aesop are currently a bit idiosyncratic. For example, they may contain the aesop_cases tactic, which is a slight variation of the standard cases. Additionally, Aesop occasionally generates buggy scripts which do not solve the goal. We hope to eventually fix these issues; until then, you may have to lightly adjust the proof scripts by hand.


To see how Aesop proves a goal -- or why it doesn't prove a goal, or why it's slow to prove a goal -- it is useful to see what it's doing. To that end, you can enable various tracing options. These use the usual syntax, e.g.

set_option trace.aesop true

The main options are:

  • trace.aesop: print a step-by-step log of which goals Aesop tried to solve, which rules it tried to apply (successfully or unsuccessfully), etc.
  • trace.aesop.ruleSet: print the rule set used for an Aesop call.
  • trace.aesop.proof: if Aesop is successful, print the proof that was generated (as a Lean term). You should be able to copy-and-paste this proof to replace Aesop.


To get an idea of where Aesop spends its time, use

set_option trace.aesop.stats true

Aesop then prints some statistics about this particular Aesop run.

To get statistics for multiple Aesop invocations, activate the option aesop.collectStats for the relevant files (or for certain invocations) and run the command #aesop_stats in a file which imports all relevant files. E.g. to evaluate Aesop's performance in Mathlib, set the option aesop.collectStats in Mathlib's lakefile.lean, recompile Mathlib from scratch and create a new Lean file with contents

import Mathlib


You can also activate the profiler option, which augments the trace produced by trace.aesop with information about how much time each step took. Note that only the timing information pertaining to goal expansions and rule applications is relevant. Other timings, such as those attached to new rapps and goals, are just artefacts of the Lean tracing API.

Checking Internal Invariants

If you encounter behaviour that looks like an internal error in Aesop, it may help to set the option aesop.check.all (or the more fine-grained aesop.check.* options). This makes Aesop check various invariants while the tactic is running. These checks are somewhat expensive, so remember to unset the option after you've reported the bug.

Handling Metavariables

Rules which create metavariables must be handled specially by Aesop. For example, suppose we register transitivity of < as an Aesop rule. Then we may get a goal state of this form:

n k : Nat
⊢ n < ?m

n k : Nat
⊢ ?m < k

We may now solve the first goal by applying different rules. We could, for example, apply the theorem ∀ n, n < n + 1. We could also use an assumption n < a. Both proofs close the first goal, but crucially, they modify the second goal: in the first case, it becomes n + 1 < k; in the second case, a < k. And of course one of these could be provable while the other is not. In other words, the second subgoal now depends on the proof of the first subgoal (whereas usually we don't care how a goal was proven, only that it was proven). Aesop could also decide to work on the second subgoal first, in which case the situation is symmetric.

Due to this dependency, Aesop in effect treats the instantiations of the second subgoal as additional goals. Thus, when we apply the theorem ∀ n, n < n + 1, which closes the first goal, Aesop realises that because this theorem was applied, we must now prove n + 1 < k as well. So it adds this goal as an additional subgoal of the rule application ∀ n, n < n + 1 (which otherwise would not have any subgoals). Similarly, when the assumption n < a is applied, its rule application gains an additional subgoal a < k.

This mechanism makes sure that we consider all potential proofs. The downside is that it's quite explosive: when there are multiple metavariables in multiple goals, which Aesop may visit in any order, Aesop may spend a lot of time copying goals with shared metavariables. It may even try to prove the same goal more than once since different rules may yield the same metavariable instantiations. For these reasons, rules which create metavariables are best kept out of the global rule set and added to individual Aesop calls on an ad-hoc basis.

It is also worth noting that when a safe rule assigns a metavariable, it is treated as an unsafe rule (with success probability 90%). This is because assigning metavariables is almost never safe, for the same reason as above: the usually perfectly safe rule ∀ n, n < n + 1 would, if treated as safe, force us to commit to one particular instantiation of the metavariable ?m.

For more details on the handling of metavariables, see the Aesop paper.