# Sigma Types

A Sigma type is a type of (dependent) tuples. If p_1, … p_n are named or unnamed parameters, then \Sigma p_1 … p_n is also a type. If A_i has type \Type p_i h_i, then the type of the Sigma type is \Type p_max h_max, where p_max is the maximum of p_1, … p_n and h_max is the maximum of h_1, … h_n.

An expression of the form \Sigma p_1 … p_n (x_1 … x_k : A) q_1 … q_m is equivalent to \Sigma p_1 … p_n (x_1 : A) … (x_k : A) q_1 … q_m.

If a_i is an expression of type A_i[a_1/x_1, … a_{i-1}/x_{i-1}], then (a_1, … a_n) is an expression of type \Sigma (x_1 : A_1) … (x_n : A_n). Note that the typechecker often cannot infer the correct type of such an expression. If the typechecker does not know it already, it always tries to guess a non-dependent version. In case the typechecker fails to infer the type, it should be specified explicitly: ((a_1, … a_n) : \Sigma (x_1 : A_1) … (x_n : A_n)). You can also explicitly specify the type of each field: (b_1 : B_1, … b_n : B_n), however in this case B_i cannot refer to previous parameters, therefore this can only be used to define non-dependent Sigma types.

If p is an expression of type \Sigma (x_1 : A_1) … (x_n : A_n) and 1 ≤ i ≤ n, then p.i is an expression of type A_i[p.1/x_1, … p_{i-1}/x_{i-1}].

An expression of the form (a_1, … a_n).i reduces to a_i.

An expression of the form (p.1, … p.n) is equivalent to p (eta equivalence for Sigma types).

A field can be marked as a property with the following syntax: \Sigma p_1 … (\property p_i) … p_n. Properties work just like record properties. That is, the type of a property must live in \Prop and (a_1, … a_n).i does not evaluate if the i-th field is a property.