Generate a node with a default (Arbitrary or database-provided) key

a <- node @A () mempty

Generate a node with an explictly-given key

let someKey = UUID.fromString "..."
a <- nodeKeyed @A someKey () mempty

Options for generating an individual node

NodeOptions can be created and combined with the Monoidal operations (<>) and mempty.

a1 <- node @A () mempty
a2 <- node @A () $ edit $ \a -> a { someField = True }
a3 <- node @A () $ ensure $ (== True) . someField

The Semigroup orders the operations from right to left. For example, edit z <> ensure y <> edit x first performs edit x, then fails if the value does not satisfy assertion y, then performs edit z.

Modify the node after it's been generated

a <- node @A () $ edit $ \a -> a { someField = True }

Require a node to satisfy the specified predicate

a <- node @A () $ ensure $ (== True) . someField

N.B. ensuring a condition that is infrequently met can be innefficient.