Tactics
In this chapter, we describe an alternative approach to constructing proofs, using tactics. A proof term is a representation of a mathematical proof; tactics are commands, or instructions, that describe how to build such a proof. Informally, you might begin a mathematical proof by saying "to prove the forward direction, unfold the definition, apply the previous lemma, and simplify." Just as these are instructions that tell the reader how to find the relevant proof, tactics are instructions that tell Lean how to construct a proof term. They naturally support an incremental style of writing proofs, in which you decompose a proof and work on goals one step at a time.
We will describe proofs that consist of sequences of tactics as "tacticstyle" proofs, to contrast with the ways of writing proof terms we have seen so far, which we will call "termstyle" proofs. Each style has its own advantages and disadvantages. For example, tacticstyle proofs can be harder to read, because they require the reader to predict or guess the results of each instruction. But they can also be shorter and easier to write. Moreover, tactics offer a gateway to using Lean's automation, since automated procedures are themselves tactics.
Entering Tactic Mode
Conceptually, stating a theorem or introducing a have
statement
creates a goal, namely, the goal of constructing a term with the
expected type. For example, the following creates the goal of
constructing a term of type p ∧ q ∧ p
, in a context with constants
p q : Prop
, hp : p
and hq : q
:
theorem test (p q : Prop) (hp : p) (hq : q) : p ∧ q ∧ p :=
sorry
You can write this goal as follows:
p : Prop, q : Prop, hp : p, hq : q ⊢ p ∧ q ∧ p
Indeed, if you replace the "sorry" by an underscore in the example above, Lean will report that it is exactly this goal that has been left unsolved.
Ordinarily, you meet such a goal by writing an explicit term. But
wherever a term is expected, Lean allows us to insert instead a by <tactics>
block, where <tactics>
is a sequence of commands,
separated by semicolons or line breaks. You can prove the theorem above
in that way:
theorem test (p q : Prop) (hp : p) (hq : q) : p ∧ q ∧ p :=
by apply And.intro
exact hp
apply And.intro
exact hq
exact hp
We often put the by
keyword on the preceding line, and write the
example above as:
theorem test (p q : Prop) (hp : p) (hq : q) : p ∧ q ∧ p := by
apply And.intro
exact hp
apply And.intro
exact hq
exact hp
The apply
tactic applies an expression, viewed as denoting a
function with zero or more arguments. It unifies the conclusion with
the expression in the current goal, and creates new goals for the
remaining arguments, provided that no later arguments depend on
them. In the example above, the command apply And.intro
yields two
subgoals:
case left
p q : Prop
hp : p
hq : q
⊢ p
case right
p q : Prop
hp : p
hq : q
⊢ q ∧ p
The first goal is met with the command exact hp
. The exact
command is just a variant of apply
which signals that the
expression given should fill the goal exactly. It is good form to use
it in a tactic proof, since its failure signals that something has
gone wrong. It is also more robust than apply
, since the
elaborator takes the expected type, given by the target of the goal,
into account when processing the expression that is being applied. In
this case, however, apply
would work just as well.
You can see the resulting proof term with the #print
command:
theorem test (p q : Prop) (hp : p) (hq : q) : p ∧ q ∧ p := by
apply And.intro
exact hp
apply And.intro
exact hq
exact hp
#print test
You can write a tactic script incrementally. In VS Code, you can open
a window to display messages by pressing CtrlShiftEnter
, and
that window will then show you the current goal whenever the cursor is
in a tactic block. In Emacs, you can see the goal at the end of any
line by pressing Cc Cg
, or see the remaining goal in an
incomplete proof by putting the cursor after the first character of
the last tactic. If the proof is incomplete, the token by
is
decorated with a red squiggly line, and the error message contains the
remaining goals.
Tactic commands can take compound expressions, not just single identifiers. The following is a shorter version of the preceding proof:
theorem test (p q : Prop) (hp : p) (hq : q) : p ∧ q ∧ p := by
apply And.intro hp
exact And.intro hq hp
Unsurprisingly, it produces exactly the same proof term.
theorem test (p q : Prop) (hp : p) (hq : q) : p ∧ q ∧ p := by
apply And.intro hp
exact And.intro hq hp
#print test
Multiple tactic applications can be written in a single line by concatenating with a semicolon.
theorem test (p q : Prop) (hp : p) (hq : q) : p ∧ q ∧ p := by
apply And.intro hp; exact And.intro hq hp
Tactics that may produce multiple subgoals often tag them. For
example, the tactic apply And.intro
tagged the first subgoal as
left
, and the second as right
. In the case of the apply
tactic, the tags are inferred from the parameters' names used in the
And.intro
declaration. You can structure your tactics using the
notation case <tag> => <tactics>
. The following is a structured
version of our first tactic proof in this chapter.
theorem test (p q : Prop) (hp : p) (hq : q) : p ∧ q ∧ p := by
apply And.intro
case left => exact hp
case right =>
apply And.intro
case left => exact hq
case right => exact hp
You can solve the subgoal right
before left
using the case
notation:
theorem test (p q : Prop) (hp : p) (hq : q) : p ∧ q ∧ p := by
apply And.intro
case right =>
apply And.intro
case left => exact hq
case right => exact hp
case left => exact hp
Note that Lean hides the other goals inside the case
block. We say
it is "focusing" on the selected goal. Moreover, Lean flags an error
if the selected goal is not fully solved at the end of the case
block.
For simple subgoals, it may not be worth selecting a subgoal using its
tag, but you may still want to structure the proof. Lean also provides
the "bullet" notation . <tactics>
(or · <tactics>
) for
structuring proof.
theorem test (p q : Prop) (hp : p) (hq : q) : p ∧ q ∧ p := by
apply And.intro
. exact hp
. apply And.intro
. exact hq
. exact hp
Basic Tactics
In addition to apply
and exact
, another useful tactic is
intro
, which introduces a hypothesis. What follows is an example
of an identity from propositional logic that we proved in a previous
chapter, now proved using tactics.
example (p q r : Prop) : p ∧ (q ∨ r) ↔ (p ∧ q) ∨ (p ∧ r) := by
apply Iff.intro
. intro h
apply Or.elim (And.right h)
. intro hq
apply Or.inl
apply And.intro
. exact And.left h
. exact hq
. intro hr
apply Or.inr
apply And.intro
. exact And.left h
. exact hr
. intro h
apply Or.elim h
. intro hpq
apply And.intro
. exact And.left hpq
. apply Or.inl
exact And.right hpq
. intro hpr
apply And.intro
. exact And.left hpr
. apply Or.inr
exact And.right hpr
The intro
command can more generally be used to introduce a variable of any type:
example (α : Type) : α → α := by
intro a
exact a
example (α : Type) : ∀ x : α, x = x := by
intro x
exact Eq.refl x
You can use it to introduce several variables:
example : ∀ a b c : Nat, a = b → a = c → c = b := by
intro a b c h₁ h₂
exact Eq.trans (Eq.symm h₂) h₁
As the apply
tactic is a command for constructing function
applications interactively, the intro
tactic is a command for
constructing function abstractions interactively (i.e., terms of the
form fun x => e
). As with lambda abstraction notation, the
intro
tactic allows us to use an implicit match
.
example (α : Type) (p q : α → Prop) : (∃ x, p x ∧ q x) → ∃ x, q x ∧ p x := by
intro ⟨w, hpw, hqw⟩
exact ⟨w, hqw, hpw⟩
You can also provide multiple alternatives like in the match
expression.
example (α : Type) (p q : α → Prop) : (∃ x, p x ∨ q x) → ∃ x, q x ∨ p x := by
intro
 ⟨w, Or.inl h⟩ => exact ⟨w, Or.inr h⟩
 ⟨w, Or.inr h⟩ => exact ⟨w, Or.inl h⟩
The intros
tactic can be used without any arguments, in which
case, it chooses names and introduces as many variables as it can. You
will see an example of this in a moment.
The assumption
tactic looks through the assumptions in context of
the current goal, and if there is one matching the conclusion, it
applies it.
example (x y z w : Nat) (h₁ : x = y) (h₂ : y = z) (h₃ : z = w) : x = w := by
apply Eq.trans h₁
apply Eq.trans h₂
assumption  applied h₃
It will unify metavariables in the conclusion if necessary:
example (x y z w : Nat) (h₁ : x = y) (h₂ : y = z) (h₃ : z = w) : x = w := by
apply Eq.trans
assumption  solves x = ?b with h₁
apply Eq.trans
assumption  solves y = ?h₂.b with h₂
assumption  solves z = w with h₃
The following example uses the intros
command to introduce the three variables and two hypotheses automatically:
example : ∀ a b c : Nat, a = b → a = c → c = b := by
intros
apply Eq.trans
apply Eq.symm
assumption
assumption
Note that names automatically generated by Lean are inaccessible by default. The motivation is to
ensure your tactic proofs do not rely on automatically generated names, and are consequently more robust.
However, you can use the combinator unhygienic
to disable this restriction.
example : ∀ a b c : Nat, a = b → a = c → c = b := by unhygienic
intros
apply Eq.trans
apply Eq.symm
exact a_2
exact a_1
You can also use the rename_i
tactic to rename the most recent inaccessible names in your context.
In the following example, the tactic rename_i h1 _ h2
renames two of the last three hypotheses in
your context.
example : ∀ a b c d : Nat, a = b → a = d → a = c → c = b := by
intros
rename_i h1 _ h2
apply Eq.trans
apply Eq.symm
exact h2
exact h1
The rfl
tactic is syntactic sugar for exact rfl
.
example (y : Nat) : (fun x : Nat => 0) y = 0 :=
by rfl
The repeat
combinator can be used to apply a tactic several times.
example : ∀ a b c : Nat, a = b → a = c → c = b := by
intros
apply Eq.trans
apply Eq.symm
repeat assumption
Another tactic that is sometimes useful is the revert
tactic,
which is, in a sense, an inverse to intro
.
example (x : Nat) : x = x := by
revert x
 goal is ⊢ ∀ (x : Nat), x = x
intro y
 goal is y : Nat ⊢ y = y
rfl
Moving a hypothesis into the goal yields an implication:
example (x y : Nat) (h : x = y) : y = x := by
revert h
 goal is x y : Nat ⊢ x = y → y = x
intro h₁
 goal is x y : Nat, h₁ : x = y ⊢ y = x
apply Eq.symm
assumption
But revert
is even more clever, in that it will revert not only an
element of the context but also all the subsequent elements of the
context that depend on it. For example, reverting x
in the example
above brings h
along with it:
example (x y : Nat) (h : x = y) : y = x := by
revert x
 goal is y : Nat ⊢ ∀ (x : Nat), x = y → y = x
intros
apply Eq.symm
assumption
You can also revert multiple elements of the context at once:
example (x y : Nat) (h : x = y) : y = x := by
revert x y
 goal is ⊢ ∀ (x y : Nat), x = y → y = x
intros
apply Eq.symm
assumption
You can only revert
an element of the local context, that is, a
local variable or hypothesis. But you can replace an arbitrary
expression in the goal by a fresh variable using the generalize
tactic.
example : 3 = 3 := by
generalize 3 = x
 goal is x : Nat ⊢ x = x
revert x
 goal is ⊢ ∀ (x : Nat), x = x
intro y
 goal is y : Nat ⊢ y = y
rfl
The mnemonic in the notation above is that you are generalizing the
goal by setting 3
to an arbitrary variable x
. Be careful: not
every generalization preserves the validity of the goal. Here,
generalize
replaces a goal that could be proved using
rfl
with one that is not provable:
example : 2 + 3 = 5 := by
generalize 3 = x
 goal is x : Nat ⊢ 2 + x = 5
admit
In this example, the admit
tactic is the analogue of the sorry
proof term. It closes the current goal, producing the usual warning
that sorry
has been used. To preserve the validity of the previous
goal, the generalize
tactic allows us to record the fact that
3
has been replaced by x
. All you need to do is to provide a
label, and generalize
uses it to store the assignment in the local
context:
example : 2 + 3 = 5 := by
generalize h : 3 = x
 goal is x : Nat, h : 3 = x ⊢ 2 + x = 5
rw [← h]
Here the rewrite
tactic, abbreviated rw
, uses h
to replace
x
by 3
again. The rewrite
tactic will be discussed below.
More Tactics
Some additional tactics are useful for constructing and destructing
propositions and data. For example, when applied to a goal of the form
p ∨ q
, you use tactics such as apply Or.inl
and apply Or.inr
. Conversely, the cases
tactic can be used to decompose a
disjunction.
example (p q : Prop) : p ∨ q → q ∨ p := by
intro h
cases h with
 inl hp => apply Or.inr; exact hp
 inr hq => apply Or.inl; exact hq
Note that the syntax is similar to the one used in match
expressions.
The new subgoals can be solved in any order.
example (p q : Prop) : p ∨ q → q ∨ p := by
intro h
cases h with
 inr hq => apply Or.inl; exact hq
 inl hp => apply Or.inr; exact hp
You can also use a (unstructured) cases
without the with
and a tactic
for each alternative.
example (p q : Prop) : p ∨ q → q ∨ p := by
intro h
cases h
apply Or.inr
assumption
apply Or.inl
assumption
The (unstructured) cases
is particularly useful when you can close several
subgoals using the same tactic.
example (p : Prop) : p ∨ p → p := by
intro h
cases h
repeat assumption
You can also use the combinator tac1 <;> tac2
to apply tac2
to each
subgoal produced by tactic tac1
.
example (p : Prop) : p ∨ p → p := by
intro h
cases h <;> assumption
You can combine the unstructured cases
tactic with the case
and .
notation.
example (p q : Prop) : p ∨ q → q ∨ p := by
intro h
cases h
. apply Or.inr
assumption
. apply Or.inl
assumption
example (p q : Prop) : p ∨ q → q ∨ p := by
intro h
cases h
case inr h =>
apply Or.inl
assumption
case inl h =>
apply Or.inr
assumption
example (p q : Prop) : p ∨ q → q ∨ p := by
intro h
cases h
case inr h =>
apply Or.inl
assumption
. apply Or.inr
assumption
The cases
tactic can also be used to
decompose a conjunction.
example (p q : Prop) : p ∧ q → q ∧ p := by
intro h
cases h with
 intro hp hq => constructor; exact hq; exact hp
In this example, there is only one goal after the cases
tactic is
applied, with h : p ∧ q
replaced by a pair of assumptions,
hp : p
and hq : q
. The constructor
tactic applies the unique
constructor for conjunction, And.intro
. With these tactics, an
example from the previous section can be rewritten as follows:
example (p q r : Prop) : p ∧ (q ∨ r) ↔ (p ∧ q) ∨ (p ∧ r) := by
apply Iff.intro
. intro h
cases h with
 intro hp hqr =>
cases hqr
. apply Or.inl; constructor <;> assumption
. apply Or.inr; constructor <;> assumption
. intro h
cases h with
 inl hpq =>
cases hpq with
 intro hp hq => constructor; exact hp; apply Or.inl; exact hq
 inr hpr =>
cases hpr with
 intro hp hr => constructor; exact hp; apply Or.inr; exact hr
You will see in Chapter Inductive Types that
these tactics are quite general. The cases
tactic can be used to
decompose any element of an inductively defined type; constructor
always applies the first applicable constructor of an inductively defined type.
For example, you can use cases
and constructor
with an existential quantifier:
example (p q : Nat → Prop) : (∃ x, p x) → ∃ x, p x ∨ q x := by
intro h
cases h with
 intro x px => constructor; apply Or.inl; exact px
Here, the constructor
tactic leaves the first component of the
existential assertion, the value of x
, implicit. It is represented
by a metavariable, which should be instantiated later on. In the
previous example, the proper value of the metavariable is determined
by the tactic exact px
, since px
has type p x
. If you want
to specify a witness to the existential quantifier explicitly, you can
use the exists
tactic instead:
example (p q : Nat → Prop) : (∃ x, p x) → ∃ x, p x ∨ q x := by
intro h
cases h with
 intro x px => exists x; apply Or.inl; exact px
Here is another example:
example (p q : Nat → Prop) : (∃ x, p x ∧ q x) → ∃ x, q x ∧ p x := by
intro h
cases h with
 intro x hpq =>
cases hpq with
 intro hp hq =>
exists x
These tactics can be used on data just as well as propositions. In the next example, they are used to define functions which swap the components of the product and sum types:
def swap_pair : α × β → β × α := by
intro p
cases p
constructor <;> assumption
def swap_sum : Sum α β → Sum β α := by
intro p
cases p
. apply Sum.inr; assumption
. apply Sum.inl; assumption
Note that up to the names we have chosen for the variables, the
definitions are identical to the proofs of the analogous propositions
for conjunction and disjunction. The cases
tactic will also do a
case distinction on a natural number:
open Nat
example (P : Nat → Prop) (h₀ : P 0) (h₁ : ∀ n, P (succ n)) (m : Nat) : P m := by
cases m with
 zero => exact h₀
 succ m' => exact h₁ m'
The cases
tactic, and its companion, the induction
tactic, are discussed in greater detail in
the Tactics for Inductive Types section.
The contradiction
tactic searches for a contradiction among the hypotheses of the current goal:
example (p q : Prop) : p ∧ ¬ p → q := by
intro h
cases h
contradiction
You can also use match
in tactic blocks.
example (p q r : Prop) : p ∧ (q ∨ r) ↔ (p ∧ q) ∨ (p ∧ r) := by
apply Iff.intro
. intro h
match h with
 ⟨_, Or.inl _⟩ => apply Or.inl; constructor <;> assumption
 ⟨_, Or.inr _⟩ => apply Or.inr; constructor <;> assumption
. intro h
match h with
 Or.inl ⟨hp, hq⟩ => constructor; exact hp; apply Or.inl; exact hq
 Or.inr ⟨hp, hr⟩ => constructor; exact hp; apply Or.inr; exact hr
You can "combine" intro h
with match h ...
and write the previous examples as follows
example (p q r : Prop) : p ∧ (q ∨ r) ↔ (p ∧ q) ∨ (p ∧ r) := by
apply Iff.intro
. intro
 ⟨hp, Or.inl hq⟩ => apply Or.inl; constructor <;> assumption
 ⟨hp, Or.inr hr⟩ => apply Or.inr; constructor <;> assumption
. intro
 Or.inl ⟨hp, hq⟩ => constructor; assumption; apply Or.inl; assumption
 Or.inr ⟨hp, hr⟩ => constructor; assumption; apply Or.inr; assumption
Structuring Tactic Proofs
Tactics often provide an efficient way of building a proof, but long sequences of instructions can obscure the structure of the argument. In this section, we describe some means that help provide structure to a tacticstyle proof, making such proofs more readable and robust.
One thing that is nice about Lean's proofwriting syntax is that it is
possible to mix termstyle and tacticstyle proofs, and pass between
the two freely. For example, the tactics apply
and exact
expect arbitrary terms, which you can write using have
, show
,
and so on. Conversely, when writing an arbitrary Lean term, you can
always invoke the tactic mode by inserting a by
block. The following is a somewhat toy example:
example (p q r : Prop) : p ∧ (q ∨ r) → (p ∧ q) ∨ (p ∧ r) := by
intro h
exact
have hp : p := h.left
have hqr : q ∨ r := h.right
show (p ∧ q) ∨ (p ∧ r) by
cases hqr with
 inl hq => exact Or.inl ⟨hp, hq⟩
 inr hr => exact Or.inr ⟨hp, hr⟩
The following is a more natural example:
example (p q r : Prop) : p ∧ (q ∨ r) ↔ (p ∧ q) ∨ (p ∧ r) := by
apply Iff.intro
. intro h
cases h.right with
 inl hq => exact Or.inl ⟨h.left, hq⟩
 inr hr => exact Or.inr ⟨h.left, hr⟩
. intro h
cases h with
 inl hpq => exact ⟨hpq.left, Or.inl hpq.right⟩
 inr hpr => exact ⟨hpr.left, Or.inr hpr.right⟩
In fact, there is a show
tactic, which is similar to the
show
expression in a proof term. It simply declares the type of the
goal that is about to be solved, while remaining in tactic
mode.
example (p q r : Prop) : p ∧ (q ∨ r) ↔ (p ∧ q) ∨ (p ∧ r) := by
apply Iff.intro
. intro h
cases h.right with
 inl hq =>
show (p ∧ q) ∨ (p ∧ r)
exact Or.inl ⟨h.left, hq⟩
 inr hr =>
show (p ∧ q) ∨ (p ∧ r)
exact Or.inr ⟨h.left, hr⟩
. intro h
cases h with
 inl hpq =>
show p ∧ (q ∨ r)
exact ⟨hpq.left, Or.inl hpq.right⟩
 inr hpr =>
show p ∧ (q ∨ r)
exact ⟨hpr.left, Or.inr hpr.right⟩
The show
tactic can actually be used to rewrite a goal to something definitionally equivalent:
example (n : Nat) : n + 1 = Nat.succ n := by
show Nat.succ n = Nat.succ n
rfl
There is also a have
tactic, which introduces a new subgoal, just as when writing proof terms:
example (p q r : Prop) : p ∧ (q ∨ r) → (p ∧ q) ∨ (p ∧ r) := by
intro ⟨hp, hqr⟩
show (p ∧ q) ∨ (p ∧ r)
cases hqr with
 inl hq =>
have hpq : p ∧ q := And.intro hp hq
apply Or.inl
exact hpq
 inr hr =>
have hpr : p ∧ r := And.intro hp hr
apply Or.inr
exact hpr
As with proof terms, you can omit the label in the have
tactic, in
which case, the default label this
is used:
example (p q r : Prop) : p ∧ (q ∨ r) → (p ∧ q) ∨ (p ∧ r) := by
intro ⟨hp, hqr⟩
show (p ∧ q) ∨ (p ∧ r)
cases hqr with
 inl hq =>
have : p ∧ q := And.intro hp hq
apply Or.inl
exact this
 inr hr =>
have : p ∧ r := And.intro hp hr
apply Or.inr
exact this
The types in a have
tactic can be omitted, so you can write have hp := h.left
and have hqr := h.right
. In fact, with this
notation, you can even omit both the type and the label, in which case
the new fact is introduced with the label this
.
example (p q r : Prop) : p ∧ (q ∨ r) → (p ∧ q) ∨ (p ∧ r) := by
intro ⟨hp, hqr⟩
cases hqr with
 inl hq =>
have := And.intro hp hq
apply Or.inl; exact this
 inr hr =>
have := And.intro hp hr
apply Or.inr; exact this
Lean also has a let
tactic, which is similar to the have
tactic, but is used to introduce local definitions instead of
auxiliary facts. It is the tactic analogue of a let
in a proof
term.
example : ∃ x, x + 2 = 8 := by
let a : Nat := 3 * 2
exists a
As with have
, you can leave the type implicit by writing let a := 3 * 2
. The difference between let
and have
is that
let
introduces a local definition in the context, so that the
definition of the local declaration can be unfolded in the proof.
We have used .
to create nested tactic blocks. In a nested block,
Lean focuses on the first goal, and generates an error if it has not
been fully solved at the end of the block. This can be helpful in
indicating the separate proofs of multiple subgoals introduced by a
tactic. The notation .
is whitespace sensitive and relies on the indentation
to detect whether the tactic block ends. Alternatively, you can
define tactic blocks using curly braces and semicolons.
example (p q r : Prop) : p ∧ (q ∨ r) ↔ (p ∧ q) ∨ (p ∧ r) := by
apply Iff.intro
{ intro h;
cases h.right;
{ show (p ∧ q) ∨ (p ∧ r);
exact Or.inl ⟨h.left, ‹q›⟩ }
{ show (p ∧ q) ∨ (p ∧ r);
exact Or.inr ⟨h.left, ‹r›⟩ } }
{ intro h;
cases h;
{ show p ∧ (q ∨ r);
rename_i hpq;
exact ⟨hpq.left, Or.inl hpq.right⟩ }
{ show p ∧ (q ∨ r);
rename_i hpr;
exact ⟨hpr.left, Or.inr hpr.right⟩ } }
It is useful to use indentation to structure proof: every time a tactic
leaves more than one subgoal, we separate the remaining subgoals by
enclosing them in blocks and indenting. Thus if the application of
theorem foo
to a single goal produces four subgoals, one would
expect the proof to look like this:
apply foo
. <proof of first goal>
. <proof of second goal>
. <proof of third goal>
. <proof of final goal>
or
apply foo
case <tag of first goal> => <proof of first goal>
case <tag of second goal> => <proof of second goal>
case <tag of third goal> => <proof of third goal>
case <tag of final goal> => <proof of final goal>
or
apply foo
{ <proof of first goal> }
{ <proof of second goal> }
{ <proof of third goal> }
{ <proof of final goal> }
Tactic Combinators
Tactic combinators are operations that form new tactics from old
ones. A sequencing combinator is already implicit in the by
block:
example (p q : Prop) (hp : p) : p ∨ q :=
by apply Or.inl; assumption
Here, apply Or.inl; assumption
is functionally equivalent to a
single tactic which first applies apply Or.inl
and then applies
assumption
.
In t₁ <;> t₂
, the <;>
operator provides a parallel version of the sequencing operation:
t₁
is applied to the current goal, and then t₂
is applied to all the resulting subgoals:
example (p q : Prop) (hp : p) (hq : q) : p ∧ q :=
by constructor <;> assumption
This is especially useful when the resulting goals can be finished off in a uniform way, or, at least, when it is possible to make progress on all of them uniformly.
The first  t₁  t₂  ...  tₙ
applies each tᵢ
until one succeeds, or else fails:
example (p q : Prop) (hp : p) : p ∨ q := by
first  apply Or.inl; assumption  apply Or.inr; assumption
example (p q : Prop) (hq : q) : p ∨ q := by
first  apply Or.inl; assumption  apply Or.inr; assumption
In the first example, the left branch succeeds, whereas in the second one, it is the right one that succeeds. In the next three examples, the same compound tactic succeeds in each case.
example (p q r : Prop) (hp : p) : p ∨ q ∨ r :=
by repeat (first  apply Or.inl; assumption  apply Or.inr  assumption)
example (p q r : Prop) (hq : q) : p ∨ q ∨ r :=
by repeat (first  apply Or.inl; assumption  apply Or.inr  assumption)
example (p q r : Prop) (hr : r) : p ∨ q ∨ r :=
by repeat (first  apply Or.inl; assumption  apply Or.inr  assumption)
The tactic tries to solve the left disjunct immediately by assumption; if that fails, it tries to focus on the right disjunct; and if that doesn't work, it invokes the assumption tactic.
You will have no doubt noticed by now that tactics can fail. Indeed,
it is the "failure" state that causes the first combinator to
backtrack and try the next tactic. The try
combinator builds a
tactic that always succeeds, though possibly in a trivial way:
try t
executes t
and reports success, even if t
fails. It is
equivalent to first  t  skip
, where skip
is a tactic that does
nothing (and succeeds in doing so). In the next example, the second
constructor
succeeds on the right conjunct q ∧ r
(remember that
disjunction and conjunction associate to the right) but fails on the
first. The try
tactic ensures that the sequential composition
succeeds.
example (p q r : Prop) (hp : p) (hq : q) (hr : r) : p ∧ q ∧ r := by
constructor <;> (try constructor) <;> assumption
Be careful: repeat (try t)
will loop forever, because the inner tactic never fails.
In a proof, there are often multiple goals outstanding. Parallel
sequencing is one way to arrange it so that a single tactic is applied
to multiple goals, but there are other ways to do this. For example,
all_goals t
applies t
to all open goals:
example (p q r : Prop) (hp : p) (hq : q) (hr : r) : p ∧ q ∧ r := by
constructor
all_goals (try constructor)
all_goals assumption
In this case, the any_goals
tactic provides a more robust solution.
It is similar to all_goals
, except it succeeds if its argument
succeeds on at least one goal.
example (p q r : Prop) (hp : p) (hq : q) (hr : r) : p ∧ q ∧ r := by
constructor
any_goals constructor
any_goals assumption
The first tactic in the by
block below repeatedly splits
conjunctions:
example (p q r : Prop) (hp : p) (hq : q) (hr : r) :
p ∧ ((p ∧ q) ∧ r) ∧ (q ∧ r ∧ p) := by
repeat (any_goals constructor)
all_goals assumption
In fact, we can compress the full tactic down to one line:
example (p q r : Prop) (hp : p) (hq : q) (hr : r) :
p ∧ ((p ∧ q) ∧ r) ∧ (q ∧ r ∧ p) := by
repeat (any_goals (first  constructor  assumption))
The combinator focus t
ensures that t
only effects the current
goal, temporarily hiding the others from the scope. So, if t
ordinarily only effects the current goal, focus (all_goals t)
has
the same effect as t
.
Rewriting
The rewrite
tactic (abbreviated rw
) and the simp
tactic
were introduced briefly in Calculational Proofs. In this
section and the next, we discuss them in greater detail.
The rewrite
tactic provides a basic mechanism for applying
substitutions to goals and hypotheses, providing a convenient and
efficient way of working with equality. The most basic form of the
tactic is rewrite [t]
, where t
is a term whose type asserts an
equality. For example, t
can be a hypothesis h : x = y
in the
context; it can be a general lemma, like
add_comm : ∀ x y, x + y = y + x
, in which the rewrite tactic tries to find suitable
instantiations of x
and y
; or it can be any compound term
asserting a concrete or general equation. In the following example, we
use this basic form to rewrite the goal using a hypothesis.
example (f : Nat → Nat) (k : Nat) (h₁ : f 0 = 0) (h₂ : k = 0) : f k = 0 := by
rw [h₂]  replace k with 0
rw [h₁]  replace f 0 with 0
In the example above, the first use of rw
replaces k
with
0
in the goal f k = 0
. Then, the second one replaces f 0
with 0
. The tactic automatically closes any goal of the form
t = t
. Here is an example of rewriting using a compound expression:
example (x y : Nat) (p : Nat → Prop) (q : Prop) (h : q → x = y)
(h' : p y) (hq : q) : p x := by
rw [h hq]; assumption
Here, h hq
establishes the equation x = y
.
Multiple rewrites can be combined using the notation rw [t_1, ..., t_n]
,
which is just shorthand for rw [t_1]; ...; rw [t_n]
. The previous example can be written as follows:
example (f : Nat → Nat) (k : Nat) (h₁ : f 0 = 0) (h₂ : k = 0) : f k = 0 := by
rw [h₂, h₁]
By default, rw
uses an equation in the forward direction, matching
the lefthand side with an expression, and replacing it with the
righthand side. The notation ←t
can be used to instruct the
tactic to use the equality t
in the reverse direction.
example (f : Nat → Nat) (a b : Nat) (h₁ : a = b) (h₂ : f a = 0) : f b = 0 := by
rw [←h₁, h₂]
In this example, the term ←h₁
instructs the rewriter to replace
b
with a
. In the editors, you can type the backwards arrow as
\l
. You can also use the ascii equivalent, <
.
Sometimes the lefthand side of an identity can match more than one
subterm in the pattern, in which case the rw
tactic chooses the
first match it finds when traversing the term. If that is not the one
you want, you can use additional arguments to specify the appropriate
subterm.
example (a b c : Nat) : a + b + c = a + c + b := by
rw [Nat.add_assoc, Nat.add_comm b, ← Nat.add_assoc]
example (a b c : Nat) : a + b + c = a + c + b := by
rw [Nat.add_assoc, Nat.add_assoc, Nat.add_comm b]
example (a b c : Nat) : a + b + c = a + c + b := by
rw [Nat.add_assoc, Nat.add_assoc, Nat.add_comm _ b]
In the first example above, the first step rewrites a + b + c
to
a + (b + c)
. The next step applies commutativity to the term
b + c
; without specifying the argument, the tactic would instead rewrite
a + (b + c)
to (b + c) + a
. Finally, the last step applies
associativity in the reverse direction, rewriting a + (c + b)
to
a + c + b
. The next two examples instead apply associativity to
move the parenthesis to the right on both sides, and then switch b
and c
. Notice that the last example specifies that the rewrite
should take place on the righthand side by specifying the second
argument to Nat.add_comm
.
By default, the rewrite
tactic affects only the goal. The notation
rw [t] at h
applies the rewrite t
at hypothesis h
.
example (f : Nat → Nat) (a : Nat) (h : a + 0 = 0) : f a = f 0 := by
rw [Nat.add_zero] at h
rw [h]
The first step, rw [Nat.add_zero] at h
, rewrites the hypothesis a + 0 = 0
to a = 0
.
Then the new hypothesis a = 0
is used to rewrite the goal to f 0 = f 0
.
The rewrite
tactic is not restricted to propositions.
In the following example, we use rw [h] at t
to rewrite the hypothesis t : Tuple α n
to t : Tuple α 0
.
def Tuple (α : Type) (n : Nat) :=
{ as : List α // as.length = n }
example (n : Nat) (h : n = 0) (t : Tuple α n) : Tuple α 0 := by
rw [h] at t
exact t
Using the Simplifier
Whereas rewrite
is designed as a surgical tool for manipulating a
goal, the simplifier offers a more powerful form of automation. A
number of identities in Lean's library have been tagged with the
[simp]
attribute, and the simp
tactic uses them to iteratively
rewrite subterms in an expression.
example (x y z : Nat) (p : Nat → Prop) (h : p (x * y))
: (x + 0) * (0 + y * 1 + z * 0) = x * y := by
simp
example (x y z : Nat) (p : Nat → Prop) (h : p (x * y))
: p ((x + 0) * (0 + y * 1 + z * 0)) := by
simp; assumption
In the first example, the lefthand side of the equality in the goal
is simplified using the usual identities involving 0 and 1, reducing
the goal to x * y = x * y
. At that point, simp
applies
reflexivity to finish it off. In the second example, simp
reduces
the goal to p (x * y)
, at which point the assumption h
finishes it off. Here are some more examples
with lists:
open List
example (xs : List Nat)
: reverse (xs ++ [1, 2, 3]) = [3, 2, 1] ++ reverse xs := by
simp
example (xs ys : List α)
: length (reverse (xs ++ ys)) = length xs + length ys := by
simp [Nat.add_comm]
As with rw
, you can use the keyword at
to simplify a hypothesis:
example (x y z : Nat) (p : Nat → Prop)
(h : p ((x + 0) * (0 + y * 1 + z * 0))) : p (x * y) := by
simp at h; assumption
Moreover, you can use a "wildcard" asterisk to simplify all the hypotheses and the goal:
attribute [local simp] Nat.mul_comm Nat.mul_assoc Nat.mul_left_comm
attribute [local simp] Nat.add_assoc Nat.add_comm Nat.add_left_comm
example (w x y z : Nat) (p : Nat → Prop)
(h : p (x * y + z * w * x)) : p (x * w * z + y * x) := by
simp at *; assumption
example (x y z : Nat) (p : Nat → Prop)
(h₁ : p (1 * x + y)) (h₂ : p (x * z * 1))
: p (y + 0 + x) ∧ p (z * x) := by
simp at * <;> constructor <;> assumption
For operations that are commutative and associative, like
multiplication on the natural numbers, the simplifier uses these two
facts to rewrite an expression, as well as left commutativity. In
the case of multiplication the latter is expressed as follows:
x * (y * z) = y * (x * z)
. The local
modifier tells the simplifier
to use these rules in the current file (or section or namespace, as
the case may be). It may seem that commutativity and
leftcommutativity are problematic, in that repeated application of
either causes looping. But the simplifier detects identities that
permute their arguments, and uses a technique known as ordered
rewriting. This means that the system maintains an internal ordering
of terms, and only applies the identity if doing so decreases the
order. With the three identities mentioned above, this has the effect
that all the parentheses in an expression are associated to the right,
and the expressions are ordered in a canonical (though somewhat
arbitrary) way. Two expressions that are equivalent up to
associativity and commutativity are then rewritten to the same
canonical form.
attribute [local simp] Nat.mul_comm Nat.mul_assoc Nat.mul_left_comm
attribute [local simp] Nat.add_assoc Nat.add_comm Nat.add_left_comm
example (w x y z : Nat) (p : Nat → Prop)
: x * y + z * w * x = x * w * z + y * x := by
simp
example (w x y z : Nat) (p : Nat → Prop)
(h : p (x * y + z * w * x)) : p (x * w * z + y * x) := by
simp; simp at h; assumption
As with rewrite
, you can send simp
a list of facts to use,
including general lemmas, local hypotheses, definitions to unfold, and
compound expressions. The simp
tactic also recognizes the ←t
syntax that rewrite
does. In any case, the additional rules are
added to the collection of identities that are used to simplify a
term.
def f (m n : Nat) : Nat :=
m + n + m
example {m n : Nat} (h : n = 1) (h' : 0 = m) : (f m n) = n := by
simp [h, ←h', f]
A common idiom is to simplify a goal using local hypotheses:
example (f : Nat → Nat) (k : Nat) (h₁ : f 0 = 0) (h₂ : k = 0) : f k = 0 := by
simp [h₁, h₂]
To use all the hypotheses present in the local context when
simplifying, we can use the wildcard symbol, *
:
example (f : Nat → Nat) (k : Nat) (h₁ : f 0 = 0) (h₂ : k = 0) : f k = 0 := by
simp [*]
Here is another example:
example (u w x y z : Nat) (h₁ : x = y + z) (h₂ : w = u + x)
: w = z + y + u := by
simp [*, Nat.add_assoc, Nat.add_comm, Nat.add_left_comm]
The simplifier will also do propositional rewriting. For example,
using the hypothesis p
, it rewrites p ∧ q
to q
and p ∨ q
to true
, which it then proves trivially. Iterating such
rewrites produces nontrivial propositional reasoning.
example (p q : Prop) (hp : p) : p ∧ q ↔ q := by
simp [*]
example (p q : Prop) (hp : p) : p ∨ q := by
simp [*]
example (p q r : Prop) (hp : p) (hq : q) : p ∧ (q ∨ r) := by
simp [*]
The next example simplifies all the hypotheses, and then uses them to prove the goal.
example (u w x x' y y' z : Nat) (p : Nat → Prop)
(h₁ : x + 0 = x') (h₂ : y + 0 = y')
: x + y + 0 = x' + y' := by
simp at *
simp [*]
One thing that makes the simplifier especially useful is that its capabilities can grow as a library develops. For example, suppose we define a list operation that symmetrizes its input by appending its reversal:
def mk_symm (xs : List α) :=
xs ++ xs.reverse
Then for any list xs
, reverse (mk_symm xs)
is equal to mk_symm xs
,
which can easily be proved by unfolding the definition:
def mk_symm (xs : List α) :=
xs ++ xs.reverse
theorem reverse_mk_symm (xs : List α)
: (mk_symm xs).reverse = mk_symm xs := by
simp [mk_symm]
We can now use this theorem to prove new results:
def mk_symm (xs : List α) :=
xs ++ xs.reverse
theorem reverse_mk_symm (xs : List α)
: (mk_symm xs).reverse = mk_symm xs := by
simp [mk_symm]
example (xs ys : List Nat)
: (xs ++ mk_symm ys).reverse = mk_symm ys ++ xs.reverse := by
simp [reverse_mk_symm]
example (xs ys : List Nat) (p : List Nat → Prop)
(h : p (xs ++ mk_symm ys).reverse)
: p (mk_symm ys ++ xs.reverse) := by
simp [reverse_mk_symm] at h; assumption
But using reverse_mk_symm
is generally the right thing to do, and
it would be nice if users did not have to invoke it explicitly. You can
achieve that by marking it as a simplification rule when the theorem
is defined:
def mk_symm (xs : List α) :=
xs ++ xs.reverse
@[simp] theorem reverse_mk_symm (xs : List α)
: (mk_symm xs).reverse = mk_symm xs := by
simp [mk_symm]
example (xs ys : List Nat)
: (xs ++ mk_symm ys).reverse = mk_symm ys ++ xs.reverse := by
simp
example (xs ys : List Nat) (p : List Nat → Prop)
(h : p (xs ++ mk_symm ys).reverse)
: p (mk_symm ys ++ xs.reverse) := by
simp at h; assumption
The notation @[simp]
declares reverse_mk_symm
to have the
[simp]
attribute, and can be spelled out more explicitly:
def mk_symm (xs : List α) :=
xs ++ xs.reverse
theorem reverse_mk_symm (xs : List α)
: (mk_symm xs).reverse = mk_symm xs := by
simp [mk_symm]
attribute [simp] reverse_mk_symm
example (xs ys : List Nat)
: (xs ++ mk_symm ys).reverse = mk_symm ys ++ xs.reverse := by
simp
example (xs ys : List Nat) (p : List Nat → Prop)
(h : p (xs ++ mk_symm ys).reverse)
: p (mk_symm ys ++ xs.reverse) := by
simp at h; assumption
The attribute can also be applied any time after the theorem is declared:
def mk_symm (xs : List α) :=
xs ++ xs.reverse
theorem reverse_mk_symm (xs : List α)
: (mk_symm xs).reverse = mk_symm xs := by
simp [mk_symm]
example (xs ys : List Nat)
: (xs ++ mk_symm ys).reverse = mk_symm ys ++ xs.reverse := by
simp [reverse_mk_symm]
attribute [simp] reverse_mk_symm
example (xs ys : List Nat) (p : List Nat → Prop)
(h : p (xs ++ mk_symm ys).reverse)
: p (mk_symm ys ++ xs.reverse) := by
simp at h; assumption
Once the attribute is applied, however, there is no way to permanently
remove it; it persists in any file that imports the one where the
attribute is assigned. As we will discuss further in
Attributes, one can limit the scope of an attribute to the
current file or section using the local
modifier:
def mk_symm (xs : List α) :=
xs ++ xs.reverse
theorem reverse_mk_symm (xs : List α)
: (mk_symm xs).reverse = mk_symm xs := by
simp [mk_symm]
section
attribute [local simp] reverse_mk_symm
example (xs ys : List Nat)
: (xs ++ mk_symm ys).reverse = mk_symm ys ++ xs.reverse := by
simp
example (xs ys : List Nat) (p : List Nat → Prop)
(h : p (xs ++ mk_symm ys).reverse)
: p (mk_symm ys ++ xs.reverse) := by
simp at h; assumption
end
Outside the section, the simplifier will no longer use
reverse_mk_symm
by default.
Note that the various simp
options we have discussed  giving an
explicit list of rules, and using at
to specify the location  can be combined,
but the order they are listed is rigid. You can see the correct order
in an editor by placing the cursor on the simp
identifier to see
the documentation string that is associated with it.
There are two additional modifiers that are useful. By default,
simp
includes all theorems that have been marked with the
attribute [simp]
. Writing simp only
excludes these defaults,
allowing you to use a more explicitly crafted list of
rules. In the examples below, the minus sign and
only
are used to block the application of reverse_mk_symm
.
def mk_symm (xs : List α) :=
xs ++ xs.reverse
@[simp] theorem reverse_mk_symm (xs : List α)
: (mk_symm xs).reverse = mk_symm xs := by
simp [mk_symm]
example (xs ys : List Nat) (p : List Nat → Prop)
(h : p (xs ++ mk_symm ys).reverse)
: p (mk_symm ys ++ xs.reverse) := by
simp at h; assumption
example (xs ys : List Nat) (p : List Nat → Prop)
(h : p (xs ++ mk_symm ys).reverse)
: p ((mk_symm ys).reverse ++ xs.reverse) := by
simp [reverse_mk_symm] at h; assumption
example (xs ys : List Nat) (p : List Nat → Prop)
(h : p (xs ++ mk_symm ys).reverse)
: p ((mk_symm ys).reverse ++ xs.reverse) := by
simp only [List.reverse_append] at h; assumption
The simp
tactic has many configuration options. For example, we can enable contextual simplifications as follows.
example : if x = 0 then y + x = y else x ≠ 0 := by
simp (config := { contextual := true })
when contextual := true
, simp
uses the fact that x = 0
when simplifying y + x = y
, and
x ≠ 0
when simplifying the other branch. Here is another example.
example : ∀ (x : Nat) (h : x = 0), y + x = y := by
simp (config := { contextual := true })
Another useful configuration option is arith := true
which enables arithmetical simplifications. It is so useful
that simp_arith
is a shorthand for simp (config := { arith := true })
.
example : 0 < 1 + x ∧ x + y + 2 ≥ y + 1 := by
simp_arith
Split Tactic
The split
tactic is useful for breaking nested ifthenelse
and match
expressions in cases.
For a match
expression with n
cases, the split
tactic generates at most n
subgoals. Here is an example.
def f (x y z : Nat) : Nat :=
match x, y, z with
 5, _, _ => y
 _, 5, _ => y
 _, _, 5 => y
 _, _, _ => 1
example (x y z : Nat) : x ≠ 5 → y ≠ 5 → z ≠ 5 → z = w → f x y w = 1 := by
intros
simp [f]
split
. contradiction
. contradiction
. contradiction
. rfl
We can compress the tactic proof above as follows.
def f (x y z : Nat) : Nat :=
match x, y, z with
 5, _, _ => y
 _, 5, _ => y
 _, _, 5 => y
 _, _, _ => 1
example (x y z : Nat) : x ≠ 5 → y ≠ 5 → z ≠ 5 → z = w → f x y w = 1 := by
intros; simp [f]; split <;> first  contradiction  rfl
The tactic split <;> first  contradiction  rfl
first applies the split
tactic,
and then for each generated goal it tries contradiction
, and then rfl
if contradiction
fails.
Like simp
, we can apply split
to a particular hypothesis.
def g (xs ys : List Nat) : Nat :=
match xs, ys with
 [a, b], _ => a+b+1
 _, [b, c] => b+1
 _, _ => 1
example (xs ys : List Nat) (h : g xs ys = 0) : False := by
simp [g] at h; split at h <;> simp_arith at h
Extensible Tactics
In the following example, we define the notation triv
using the command syntax
.
Then, we use the command macro_rules
to specify what should
be done when triv
is used. You can provide different expansions, and the tactic
interpreter will try all of them until one succeeds.
 Define a new tactic notation
syntax "triv" : tactic
macro_rules
 `(tactic triv) => `(tactic assumption)
example (h : p) : p := by
triv
 You cannot prove the following theorem using `triv`
 example (x : α) : x = x := by
 triv
 Let's extend `triv`. The tactic interpreter
 tries all possible macro extensions for `triv` until one succeeds
macro_rules
 `(tactic triv) => `(tactic rfl)
example (x : α) : x = x := by
triv
example (x : α) (h : p) : x = x ∧ p := by
apply And.intro <;> triv
 We now add a (recursive) extension
macro_rules  `(tactic triv) => `(tactic apply And.intro <;> triv)
example (x : α) (h : p) : x = x ∧ p := by
triv
Exercises

Go back to the exercises in Chapter Propositions and Proofs and Chapter Quantifiers and Equality and redo as many as you can now with tactic proofs, using also
rw
andsimp
as appropriate. 
Use tactic combinators to obtain a one line proof of the following:
example (p q r : Prop) (hp : p)
: (p ∨ q ∨ r) ∧ (q ∨ p ∨ r) ∧ (q ∨ r ∨ p) := by
admit