dependent type (Rev #2, changes)

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A dependent type is a family of types indexed by - “depending on” - values of another type.

Given a type $A$ in a universe of types $\mathcal{U}$, a dependent product type (or ‘pi-type’) $B$ is a family of types:

$\Pi_{(a:A)}B(a)$

Similarly, a dependent sum type (or ‘sigma-type’) $B$ is a family of types:

$\Sigma_{(a:A)}B(a)$