# Rewriting¶

Rewrite rules allow you to extend Agda’s evaluation relation with new computation rules.

Note

This page is about the `--rewriting`

option and the
associated REWRITE builtin. You might be
looking for the documentation on the rewrite construct instead.

## Rewrite rules by example¶

To enable rewrite rules, you should run Agda with the flag `--rewriting`

and import the modules `Agda.Builtin.Equality`

and `Agda.Builtin.Equality.Rewrite`

:

```
{-# OPTIONS --rewriting #-}
module language.rewriting where
open import Agda.Builtin.Equality
open import Agda.Builtin.Equality.Rewrite
```

### Overlapping pattern matching¶

To start, let’s look at an example where rewrite rules can solve a
problem that is encountered by almost every newcomer to Agda. This
problem usually pops up as the question why `0 + m`

computes to
`m`

, but `m + 0`

does not (and similarly, `(suc m) + n`

computes
to `suc (m + n)`

but `m + (suc n)`

does not). This problem
manifests itself for example when trying to prove commutativity of `_+_`

:

```
+comm : m + n ≡ n + m
+comm {m = zero} = refl
+comm {m = suc m} = cong suc (+comm {m = m})
```

Here, Agda complains that `n != n + zero of type Nat`

. The usual way
to solve this problem is by proving the equations `m + 0 ≡ m`

and
`m + (suc n) ≡ suc (m + n)`

and using an explicit `rewrite`

statement in the main proof (N.B.: Agda’s `rewrite`

keyword should not
be confused with rewrite rules, which are added by a `REWRITE`

pragma.)

By using rewrite rules, we can simulate the solution from our paper. First, we need to prove that the equations we want hold as propositional equalities:

```
+zero : m + zero ≡ m
+zero {m = zero} = refl
+zero {m = suc m} = cong suc +zero
+suc : m + (suc n) ≡ suc (m + n)
+suc {m = zero} = refl
+suc {m = suc m} = cong suc +suc
```

Next we mark the equalities as rewrite rules with a `REWRITE`

pragma:

```
{-# REWRITE +zero +suc #-}
```

Now the proof of commutativity works exactly as we wrote it before:

```
+comm : m + n ≡ n + m
+comm {m = zero} = refl
+comm {m = suc m} = cong suc (+comm {m = m})
```

Note that there is no way to make this proof go through without
rewrite rules: it is essential that `_+_`

computes both on its first
and its second argument, but there’s no way to define `_+_`

in such a
way using Agda’s regular pattern matching.

### More examples¶

Additional examples of how to use rewrite rules can be found in a blog post by Jesper Cockx.

## General shape of rewrite rules¶

In general, an equality proof `eq`

may be registered as a rewrite
rule using the pragma `{-# REWRITE eq #-}`

, provided the following
requirements are met:

- The type of
`eq`

is of the form`eq : (x₁ : A₁) ... (xₖ : Aₖ) → f p₁ ... pₙ ≡ v`

`f`

is a postulate, a defined function symbol, or a constructor applied to fully general parameters (i.e. the parameters must be distinct variables)- Each variable
`x₁`

, …,`xₖ`

occurs at least once in a pattern position in`p₁ ... pₙ`

(see below for the definition of pattern positions) - The left-hand side
`f p₁ ... pₙ`

should be neutral, i.e. it should not reduce.

The following patterns are supported:

`x y₁ ... yₙ`

, where`x`

is a pattern variable and`y₁`

, …,`yₙ`

are distinct variables that are bound locally in the pattern`f p₁ ... pₙ`

, where`f`

is a postulate, a defined function, a constructor, or a data/record type, and`p₁`

, …,`pₙ`

are again patterns`λ x → p`

, where`p`

is again a pattern`(x : P) → Q`

, where`P`

and`Q`

are again patterns`y p₁ ... pₙ`

, where`y`

is a variable bound locally in the pattern and`p₁`

, …,`pₙ`

are again patterns`Set p`

or`Prop p`

, where`p`

is again a pattern- Any other term
`v`

(here the variables in`v`

are not considered to be in a pattern position)

Once a rewrite rule has been added, Agda automatically rewrites all
instances of the left-hand side to the corresponding instance of the
right-hand side during reduction. More precisely, a term
(definitionally equal to) `f p₁σ ... pₙσ`

is rewritten to `vσ`

,
where `σ`

is any substitution on the pattern variables `x₁`

,
… `xₖ`

.

Since rewriting happens after normal reduction, rewrite rules are only applied to terms that would otherwise be neutral.

## Confluence checking¶

Agda can optionally check confluence of rewrite rules by enabling the
`--confluence-check`

flag. Concretely, it does so by enforcing two
properties:

- For any two left-hand sides of the rewrite rules that overlap (either at the root position or at a subterm), the most general unifier of the two left-hand sides is again a left-hand side of a rewrite rule. For example, if there are two rules
`suc m + n = suc (m + n)`

and`m + suc n = suc (m + n)`

, then there should also be a rule`suc m + suc n = suc (suc (m + n))`

.- Each rewrite rule should satisfy the
triangle property: For any rewrite rule`u = w`

and any single-step parallel unfolding`u => v`

, we should have another single-step parallel unfolding`v => w`

.

There is also a flag `--local-confluence-check`

that is less
restrictive but only checks local confluence of rewrite rules. In case
the rewrite rules are terminating (currently not checked), these two
properties are equivalent.

## Advanced usage¶

Instead of importing `Agda.Builtin.Equality.Rewrite`

, a different
type may be chosen as the rewrite relation by registering it as the
`REWRITE`

builtin. For example, using the pragma ```
{-# BUILTIN
REWRITE _~_ #-}
```

registers the type `_~_`

as the rewrite
relation. To qualify as the rewrite relation, the type must take at
least two arguments, and the final two arguments should be visible.