# Higher Inductive Types

Higher inductive types generalize ordinary inductive types.
Various homotopy colimits of types and other constructions such as
truncations are higher inductive types. A specific homotopy structure of a higher inductive
type can be defined by means of *conditions* in data definitions.

## Conditions

If con is a constructor of an inductive type D, then an expression of the form
con a_1 … a_n does not reduce any further unless the definition of D contains *conditions* on con.
A condition on a constructor is a rule that says how such an expression might be reduced.
For example, one can define integers as a data type with two constructors: one for positive, and one for negative integers, and a condition on the second constructor telling positive and negative zero have to be computationally equal:

```
\data Int
| pos Nat
| neg Nat \with {
| zero => pos zero
}
```

Conditions are imposed on a constructor by defining it as a function by pattern matching. The only differences are that it is not required that all cases are covered and that pattern matching on constructors left and right of the interval type I is allowed. The general syntax is the same as for ordinary pattern matching. Either \with { | c_1 | … | c_m } or \elim x_1, … x_n { | c_1 | … | c_m } can be added after parameters of the constructor, where | c_1 | … | c_m is a list of clauses.

A constructor with conditions evaluates if its arguments match the specification in the same way as a function defined by pattern matching. This means that a function over a data type with conditions must respect the conditions, this is checked by the typechecker. For example, a function of type Int -> X must map positive and negative zero to the same value. Thus, one cannot define the following function:

```
\func f (x : Int) : Nat
| pos n => n
| neg n => suc n
```

## Higher inductive types

A higher inductive type is a data type with a constructor that has conditions of the form | left => e and | right => e’. Let us give a few examples:

```
-- Circle
\data S1
| base
| loop I \with {
| left => base
| right => base
}
-- Suspension
\data Susp (A : \Type)
| north
| south
| merid A (i : I) \elim i {
| left => north
| right => south
}
-- Propositional truncation
\data Trunc (A : \Type)
| inT A
| truncT (x y : Trunc A) (i : I) \elim i {
| left => x
| right => y
}
-- Set quotient
\data Quotient (A : \Type) (R : A -> A -> \Type)
| inQ A
| equivQ (x y : A) (R x y) (i : I) \elim i {
| left => inQ x
| right => inQ y
}
| truncQ (a a' : Quotient A R) (p p' : a = a') (i j : I) \elim i, j {
| i, left => p @ i
| i, right => p' @ i
| left, _ => a
| right, _ => a'
}
```

If X is a proposition, then, to define a function of type Trunc A -> X, it is enough to specify its value for inT a. The same works for any higher inductive type and any level. For example, to define a function Quotient A R -> X, it is enough to specify its value for inQ a and equivQ x y r i if X is a set and only for inQ a if it is a proposition. This also works for several arguments. For example, if X is a set, then, to define a function Quotient A R -> Quotient A R -> X, it is enough to specify its value for inQ a, inQ a’, inQ a, equivQ x y r i, and equivQ x y r i, inQ a.