Open union is a strong sum (existential with an evidence).

Orthogonal decomposition of a Union (t ': r) :: * -> * . Right value is returned if the Union (t ': r) :: * -> * contains t :: * -> * , and Left when it doesn't. Notice that Left value contains Union r :: * -> * , i.e. it can not contain t :: * -> * .

O(1)

Inject whole Union r into a weaker Union (any ': r) that has one more summand.

O(1)

Specialised version of prj / decomp that works on an Union '[t] :: * -> * which contains only one specific summand. Hence the absence of Maybe , and Either .

O(1)

A constraint that requires that a particular effect, eff , is a member of the type-level list effs . This is used to parameterize an Eff computation over an arbitrary list of effects, so long as eff is somewhere in the list.

For example, a computation that only needs access to a cell of mutable state containing an Integer would likely use the following type:

Member (State Integer) effs => Eff effs ()

A shorthand constraint that represents a combination of multiple Member constraints. That is, the following Members constraint:

Members '[Foo, Bar, Baz] effs

…is equivalent to the following set of Member constraints:

(Member Foo effs, Member Bar effs, Member baz effs)

Note that, since each effect is translated into a separate Member constraint, the order of the effects does not matter.

Like Member , LastMember eff effs is a constraint that requires that eff is in the type-level list effs . However, unlike Member , LastMember requires m be the final effect in effs .

Generally, this is not especially useful, since it is preferable for computations to be agnostic to the order of effects, but it is quite useful in combination with sendM or liftBase to embed ordinary monadic effects within an Eff computation.