Summary

`decide` will attempt to solve a goal if it can find an algorithm which it

can run to solve it.

Example

A term of type `DecidableEq ℕ` is an algorithm to decide whether two naturals

are equal or different. Hence, once this term is made and made into an `instance`,

the `decide` tactic can use it to solve goals of the form `a = b` or `a ≠ b`.

TacticDoc decide

NewTactic decide

Introduction

Implementing the algorithm for equality of naturals, and the proof that it is correct,

looks like this:

```

instance instDecidableEq : DecidableEq ℕ

| 0, 0 => isTrue <| by

show 0 = 0

rfl

| succ m, 0 => isFalse <| by

show succ m ≠ 0

exact succ_ne_zero m

| 0, succ n => isFalse <| by

show 0 ≠ succ n

exact zero_ne_succ n

| succ m, succ n =>

match instDecidableEq m n with

| isTrue (h : m = n) => isTrue <| by

show succ m = succ n

rw [h]

rfl

| isFalse (h : m ≠ n) => isFalse <| by

show succ m ≠ succ n

exact succ_ne_succ m n h

```

This Lean code is a formally verified algorithm for deciding equality

between two naturals. I've typed it in already, behind the scenes.

Because the algorithm is formally verified to be correct, we can

use it in Lean proofs. You can run the algorithm with the `decide` tactic.

/-- $20+20=40$ . -/

Statement : (20 : ℕ) + 20 = 40 := by

decide

Conclusion "You can read more about the `decide` tactic by clicking

on it in the top right."