This is the Overture.Basic module of the Agda Universal Algebra Library.
(See also the Equality module of the agda-algebras library.)
An Agda program typically begins by setting some options and by importing types
from existing Agda libraries. Options are specified with the OPTIONS
pragma
and control the way Agda behaves by, for example, specifying the logical axioms
and deduction rules we wish to assume when the program is type-checked to verify
its correctness. Every Agda program in agda-algebras begins with the following line.
{-# OPTIONS --without-K --exact-split --safe #-}
These options control certain foundational assumptions that Agda makes when type-checking the program to verify its correctness.
--without-K
disables
Streicher’s K axiom;
see also the
section on axiom K
in the Agda Language Reference Manual.
--exact-split
makes Agda accept only those definitions that behave like so-called
judgmental equalities. Martín Escardó explains
this by saying it “makes sure that pattern matching corresponds to Martin-Löf
eliminators;” see also the
Pattern matching and equality section
of the Agda Tools documentation.
safe
ensures that nothing is postulated outright—every non-MLTT axiom has to be
an explicit assumption (e.g., an argument to a function or module); see also
this section
of the Agda Tools documentation and the
Safe Agda section
of the Agda Language Reference.
Note that if we wish to type-check a file that imports another file that still
has some unmet proof obligations, we must replace the --safe
flag with
--allow-unsolved-metas
, but this is never done in (publicly released versions
of) the agda-algebras.
The OPTIONS
pragma is usually followed by the start of a module. For example,
the [Base.Functions.Basic][] module begins with the following line, and then a
list of imports of things used in the module.
module Overture.Basic where -- Imports from Agda and the Agda Standard Library ----------------------------------------------- open import Agda.Primitive using () renaming ( Set to Type ; lzero to ℓ₀ ) open import Data.Product using ( _,_ ; ∃ ; Σ-syntax ; _×_ ) renaming ( proj₁ to fst ; proj₂ to snd ) open import Function.Base using ( _∘_ ; id ) open import Level using ( Level ; suc ; _⊔_ ; lift ; lower ; Lift ) open import Relation.Binary using ( Decidable ) open import Relation.Binary using ( IsEquivalence ; IsPartialOrder ) open import Relation.Nullary using ( Dec ; yes ; no ; Irrelevant ) open import Relation.Binary.PropositionalEquality using ( _≡_ ; refl ; sym ; trans ) private variable α β : Level ℓ₁ : Level ℓ₁ = suc ℓ₀ -- the two element type data 𝟚 : Type ℓ₀ where 𝟎 : 𝟚 ; 𝟏 : 𝟚 -- the three element type data 𝟛 : Type ℓ₀ where 𝟎 : 𝟛 ; 𝟏 : 𝟛 ; 𝟐 : 𝟛
The definition of Σ
(and thus, of ×
) includes the fields proj₁
and proj₂
representing the first and second projections out of the product. However, we
prefer the shorter names fst
and snd
. Sometimes we prefer to denote these
projections by ∣_∣
and ∥_∥
, respectively. We define these alternative
notations for projections out of pairs as follows.
module _ {A : Type α }{B : A → Type β} where ∣_∣ : Σ[ x ∈ A ] B x → A ∣_∣ = fst ∥_∥ : (z : Σ[ a ∈ A ] B a) → B ∣ z ∣ ∥_∥ = snd infix 40 ∣_∣
Here we put the definitions inside an anonymous module, which starts with the
module
keyword followed by an underscore (instead of a module name). The
purpose is simply to move the postulated typing judgments—the “parameters”
of the module (e.g., A : Type α
)—out of the way so they don’t obfuscate
the definitions inside the module.
Let’s define some useful syntactic sugar that will make it easier to apply
symmetry and transitivity of ≡
in proofs.
_⁻¹ : {A : Type α} {x y : A} → x ≡ y → y ≡ x p ⁻¹ = sym p infix 40 _⁻¹
If we have a proof p : x ≡ y
, and we need a proof of y ≡ x
, then instead of
sym p
we can use the more intuitive p ⁻¹
. Similarly, the following syntactic
sugar makes abundant appeals to transitivity easier to stomach.
_∙_ : {A : Type α}{x y z : A} → x ≡ y → y ≡ z → x ≡ z p ∙ q = trans p q 𝑖𝑑 : (A : Type α ) → A → A 𝑖𝑑 A = λ x → x infixl 30 _∙_
infix 2 ∃-syntax ∃-syntax : ∀ {A : Type α} → (A → Type β) → Set (α ⊔ β) ∃-syntax = ∃ syntax ∃-syntax (λ x → B) = ∃[ x ∈ A ] B
The dependent function type is traditionally denoted with an uppercase pi symbol
and typically expressed as Π(x : A) B x
, or something similar. In Agda syntax,
one writes (x : A) → B x
for this dependent function type, but we can define
syntax that is closer to standard notation as follows.
Π : {A : Type α } (B : A → Type β ) → Type (α ⊔ β) Π {A = A} B = (x : A) → B x Π-syntax : (A : Type α)(B : A → Type β) → Type (α ⊔ β) Π-syntax A B = Π B syntax Π-syntax A (λ x → B) = Π[ x ∈ A ] B infix 6 Π-syntax
In the modules that follow, we will see many examples of this syntax in action.
The hierarchy of universes in Agda is structured as follows:
Type α : Type (lsuc α) , Type (lsuc α) : Type (lsuc (lsuc α)) , etc.
and so on. This means that the universe Type α
has type Type(lsuc α)
, and
Type(lsuc α)
has type Type(lsuc (lsuc α))
, and so on. It is important to
note, however, this does not imply that Type α : Type(lsuc(lsuc α))
. In other
words, Agda’s universe hierarchy is non-cumulative. This makes it possible to
treat universe levels more precisely, which is nice. On the other hand, a
non-cumulative hierarchy can sometimes make for a non-fun proof assistant.
Specifically, in certain situations, the non-cumulativity makes it unduly
difficult to convince Agda that a program or proof is correct.
Here we describe a general Lift
type that help us overcome the technical issue
described in the previous subsection. In the Lifts of algebras
section of the
Base.Algebras.Basic module we will define a couple domain-specific lifting
types which have certain properties that make them useful for resolving universe
level problems when working with algebra types.
Let us be more concrete about what is at issue here by considering a typical example. Agda will often complain with errors like the following:
Birkhoff.lagda:498,20-23
α != 𝓞 ⊔ 𝓥 ⊔ (lsuc α) when checking that the expression... has type...
This error message means that Agda encountered the universe level lsuc α
, on
line 498 (columns 20–23) of the file Birkhoff.lagda
, but was expecting a type
at level 𝓞 ⊔ 𝓥 ⊔ lsuc α
instead.
The general Lift
record type that we now describe makes such problems easier to
deal with. It takes a type inhabiting some universe and embeds it into a higher
universe and, apart from syntax and notation, it is equivalent to the Lift
type
one finds in the Level
module of the Agda Standard Library.
record Lift {𝓦 α : Level} (A : Set α) : Set (α ⊔ 𝓦) where
constructor lift
field lower : A
The point of having a ramified hierarchy of universes is to avoid Russell’s
paradox, and this would be subverted if we were to lower the universe of a type
that wasn’t previously lifted. However, we can prove that if an application of
lower
is immediately followed by an application of lift
, then the result is
the identity transformation. Similarly, lift
followed by lower
is the
identity.
lift∼lower : {A : Type α} → lift ∘ lower ≡ 𝑖𝑑 (Lift β A) lift∼lower = refl lower∼lift : {A : Type α} → (lower {α}{β}) ∘ lift ≡ 𝑖𝑑 A lower∼lift = refl
The proofs are trivial. Nonetheless, we’ll come across some holes these lemmas can fill.
We conclude this module with a definition that conveniently represents te assertion that two functions are (extensionally) the same in the sense that they produce the same output when given the same input. (We will have more to say about this notion of equality in the Base.Equality.Extensionality module.)
module _ {α : Level}{A : Type α}{β : Level}{B : A → Type β } where _≈_ : (f g : (a : A) → B a) → Type (α ⊔ β) f ≈ g = ∀ x → f x ≡ g x infix 8 _≈_ ≈IsEquivalence : IsEquivalence _≈_ IsEquivalence.refl ≈IsEquivalence = λ _ → refl IsEquivalence.sym ≈IsEquivalence f≈g = λ x → sym (f≈g x) IsEquivalence.trans ≈IsEquivalence f≈g g≈h = λ x → trans (f≈g x) (g≈h x)
The following is convenient for proving two pairs of a product type are equal using the fact that their respective components are equal.
≡-by-parts : {A : Type α}{B : Type β}{u v : A × B} → fst u ≡ fst v → snd u ≡ snd v → u ≡ v ≡-by-parts refl refl = refl
Lastly, we will use the following type (instead of subst
) to transport equality
proofs.
transport : {A : Type α } (B : A → Type β) {x y : A} → x ≡ y → B x → B y transport B refl = id