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:

  1. {-# OPTIONS --rewriting #-}
  3. module language.rewriting where
  5. open import Agda.Builtin.Equality
  6. 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 _+_:

  1. +comm : m + n ≡ n + m
  2. +comm {m = zero}  = refl
  3. +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:

  1. +zero : m + zero ≡ m
  2. +zero {m = zero}  = refl
  3. +zero {m = suc m} = cong suc +zero
  5. +suc : m + (suc n) ≡ suc (m + n)
  6. +suc {m = zero}  = refl
  7. +suc {m = suc m} = cong suc +suc

Next we mark the equalities as rewrite rules with a REWRITE pragma:

  1. {-# REWRITE +zero +suc #-}

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

  1. +comm : m + n ≡ n + m
  2. +comm {m = zero}  = refl
  3. +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:

  1. 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)).

  2. 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.