Introduction to the conversion mode tactic

Inside a tactic block, one can use the conv tactic to enter conversion mode. This mode allows one to travel into subexpressions inside assumptions and goals, even inside lambda functions and foralls, to apply targeted rewrites, simplifications, and other tactics.

This is similar to the conversion tacticals (tactic combinators) found in other theorem provers like HOL4, HOL Light or Isabelle.

Basic navigation and rewriting

As a first example, let us prove example (a b c : ℕ) : a * (b * c) = a * (c * b) (examples in this file are somewhat artificial since the ring tactic from Mathlib/Tactic/Ring.lean could finish them immediately). The naive first attempt is to enter tactic mode and try rw [mul_comm]. But this transforms the goal into b * c * a = a * (c * b), after commuting the very first multiplication appearing in the term. There are several ways to fix this issue, and one way is to use a more precise tool: the conversion mode. The following code block shows the current target after each line. Note that the target is prefixed by | where normal tactic mode shows a goal prefixed by ⊢. Both cases are still called "goals" though.

example (a b c : ℕ) : a * (b * c) = a * (c * b) := by
  conv =>          -- `| a * (b * c) = a * (c * b)`
    lhs            -- `| a * (b * c)`
    congr          -- 2 goals : `| a` and `| b * c`
    rfl            -- skip `| a` goal
    rw [mul_comm]  -- `| c * b`

The above snippet show three navigation commands:

• lhs navigates to the left-hand side of a relation (here equality), there is also a rhs navigating to the right-hand side.
• congr creates as many targets as there are arguments to the current head function (here the head function is multiplication)
• rfl goes to the next target

Once we arrive to the relevant target, we can use rw as in normal tactic mode. At the end, conv will automatically use rfl to skip the last remaining target.

The second main reason to use conversion mode is to rewrite subexpressions involving bound variables ("rewrite under binders"). Suppose we want to prove example : (fun x : ℕ => 0 + x) = (fun x => x). The naive first attempt is to enter tactic mode and try rw [zero_add]. However, this fails with a frustrating

tactic 'rewrite' failed, did not find instance of the pattern in the target expression
  0 + ?a
⊢ (fun x ↦ 0 + x) = fun x ↦ x

The solution is:

example : (fun x : ℕ => 0 + x) = (fun x => x) := by
  conv =>          -- | (fun x ↦ 0 + x) = fun x ↦ x
    lhs            -- | fun x ↦ 0 + x
    ext x          -- | 0 + x
    rw [zero_add]  -- | x

where ext is the navigation command entering inside the fun binder (the x argument is optional). Note that this example is somewhat artificial, one could also do:

example : (fun x : ℕ => 0 + x) = (fun x => x) := by
  ext
  rw [zero_add]

All of this is also available for converting a hypothesis H in the local context by using the syntax conv at H => ....

Here are a more ways to navigate expressions:

• arg i navigates to the ith explicit argument. It is like doing congr and the appropriate number of rfls for all but the ith argument.
• arg @i navigates to the ith argument, counting both explicit and implicit arguments.
• enter [...], where the ... consists of a list of arguments appropriate for arg or ext, and then runs the corresponding arg and ext commands. For example, enter [1,@2,x,3] is the same as arg 1; arg @2; ext x; arg 3.

Pattern matching

Navigation using the above commands can be tedious. One can shortcut it using pattern matching as follows:

example (a b c : ℕ) : a * (b * c) = a * (c * b) := by
  conv in b * c =>  -- | b * c
    rw [mul_comm]   -- | c * b

This in clause is short for

example (a b c : ℕ) : a * (b * c) = a * (c * b) := by
  conv =>           -- | a * (b * c) = a * (c * b)
    pattern b * c   -- | b * c
    rw [mul_comm]   -- | c * b

As usual for => block tactics, the body can be placed on a single line with tactics separated by semicolons. This yields a one-liner:

example (a b c : ℕ) : a * (b * c) = a * (c * b) := by
  conv in b * c => rw [mul_comm]

Of course placeholders are allowed:

example (a b c : ℕ) : a * (b * c) = a * (c * b) := by
  conv in _ * c => rw [mul_comm]

In all those cases, only the first match is affected. One can also specify which occurrences to convert using an occs clause, which creates goals for every matched occurrence. These can then all be handled at once using the all_goals combinator. The following performs rewriting only for the second and third occurrences of b * c:

example (b c : ℕ) :
    (b * c) * (b * c) * (b * c) = (b * c) * (c * b) * (c * b) := by
  conv in (occs := 2 3) b * c =>
    all_goals rw [mul_comm]

This can also be done using pattern and the <;> combinator, where, like in normal tactic mode, t1 <;> t2 means to run t1 and then run t2 for every goal produced by it.

example (b c : ℕ) :
    (b * c) * (b * c) * (b * c) = (b * c) * (c * b) * (c * b) := by
  conv => pattern (occs := 2 3) b * c <;> rw [mul_comm]

Sub-conversions

The conv tactic supports nested conv mode. This allows one to do a targeted rewrite using the power of conv mode and then return to the original position with the rewritten expression.

example (a b : ℕ) :
    a * b * (a * b) = b * a * (a * b) := by
  conv =>
    -- | a * b * (a * b) = b * a * (a * b)
    conv => pattern (occs := 2) a * b; rw [mul_comm]
    -- | a * b * (b * a) = b * a * (a * b)
    rw [mul_comm]
    -- | b * a * (a * b) = b * a * (a * b)

Other tactics inside conversion mode

Besides rewriting using rw, one can use simp, dsimp, change, equals, ring, norm_num, push_neg, unfold, among others.

See the conv guide for a more in-depth overview.