| Maintainer | diagrams-discuss@googlegroups.com |
|---|---|
| Safe Haskell | None |
Data.Monoid.MList
Contents
Description
Heterogeneous lists of monoids.
Heterogeneous monoidal lists
The idea of heterogeneous lists has been around for a long time. Here, we adopt heterogeneous lists where the element types are all monoids: this allows us to leave out identity values, so that a heterogeneous list containing only a single non-identity value can be created without incurring constraints due to all the other types, by leaving all the other values out.
Type class for heterogeneous monoidal lists, with a single method allowing construction of an empty list.
Methods
Accessing embedded values
The relation l :>: a holds when a is the type of an element
in l. For example, (Char ::: Int ::: Bool ::: Nil) :>: Int.
Monoid actions of heterogeneous lists
Monoidal heterogeneous lists may act on one another as you would expect, with each element in the first list acting on each in the second. Unfortunately, coding this up in type class instances is a bit fiddly.
SM, an abbreviation for "single monoid" (as opposed to a
heterogeneous list of monoids), is only used internally to help
guide instance selection when defining the action of
heterogeneous monoidal lists on each other.
Constructors
| SM m |