Variant

Variant is a sum type, i.e. a wrapper for a value which can be of different types. For instance in the following code x is a variant whose value can be an Int or a Float:

import Haskus.Utils.Variant

x :: V '[Int,Float]

The particularity is that we use a type-level list of types to control the possible value type. Compared to usual sum types (e.g. Either Int Float) it allows us to manipulate (extend/filter/etc.) the list in a type-safe way and without requiring new data types.

Recursive sum types (e.g. data List a = Nil | Cons a (List a)) based on Variant are also supported and are called EADT (for “extensible ADT”).

• 1. Introduction
• 2. Basics
• 3. Safe pattern matching
• 4. Writing generic functions using variants
• 5. Converting variants
• 6. Updating variants
• 7. Do-notation