Function check

module FunctionCheck where

import Syntax (Constant(..))

The function check governs the types of functions that our pure type system permits. This is based on CCω with only three universes:

Type is an impredicative universe at the bottom of the hierarchy (equivalent to * from the linked paper)
Kind is the first predicate universe (equivalent to □₀)
Sort is the second predicate universe (equivalent to □₁)

These universes form a hierarchy, which can be witnessed by the least-upper bound judgement ⋁:

─────────────────────  ; Type < Kind < Sort
T₀ ⋁ T₁ = max(T₀, T₁)

... where T₀ and T₁ are either Type, Kind, or Sort (though they may differ).

The function check is a judgment of the form:

c₀ ↝ c₁ : c₂

... where:

c₀ (an input constant, either Type, Kind, or Sort) is the type of the function's input type
c₁ (an input constant, either Type, Kind, or Sort) is the type of the function's output type
c₂ (an output constant, either Type, Kind, or Sort) is the type of the function's type

functionCheck :: Constant -> Constant -> Constant

Functions that return terms are impredicative:

───────────────
c ↝ Type : Type

functionCheck _c Type = Type

When c = Type you get functions from terms to terms (i.e. "term-level" functions):

──────────────────
Type ↝ Type : Type

For example, these are term-level functions permitted by the above rule:

Natural/even

λ(x : Bool) → x != False

When c = Kind you get functions from types to terms (i.e. "type-polymorphic" functions):

──────────────────
Kind ↝ Type : Type

For example, these are type-polymorphic functions permitted by the above rule:

List/head

λ(a : Type) → λ(x : a) → x

When c = Sort you get functions from sorts to terms:

──────────────────
Sort ↝ Type : Type

For example, this is a (trivial) function from a sort to a term:

λ(k : Kind) → 1

All the remaining function types are predicative:

c₀ ⋁ c₁ = c₂
────────────
c₀ ↝ c₁ : c₂

functionCheck c₀ c₁ = c₂
  where
    c₂ = max c₀ c₁

When c₀ = Kind and c₁ = Kind you get functions from types to types (i.e. "type-level" functions):

──────────────────
Kind ↝ Kind : Kind

For example, these are type-level functions permitted by the above rule:

List

λ(m : Type) → m → List m

When c₀ = Type and c₁ = Kind you get functions from terms to types (i.e. "dependent" types):

──────────────────
Type ↝ Kind : Kind

For example, this is a dependently-typed function permitted by the above rule:

λ(n : Natural) → n ≡ (n + 0)

When c₀ = Sort and c₁ = Kind you get functions from kinds to types (i.e. "kind-polymorphic" functions):

──────────────────
Sort ↝ Kind : Sort

For example, this is a kind-polymorphic function permitted by the above rules:

λ(k : Kind) → λ(a : k) → a

When c₀ = Sort and c₁ = Sort you get functions from kinds to kinds (i.e. "kind-level" functions):

──────────────────
Sort ↝ Sort : Sort

For example, this is a kind-level function permitted by the above rule:

λ(a : Kind) → a → a

You can also have sort-level dependently-typed functions:

──────────────────
Type ↝ Sort : Sort


──────────────────
Kind ↝ Sort : Sort

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