This library provides a way to describe relations between a container and its content, and use that description to manipulate arbitrary data more easily.
There are two kinds of relations between a container and its content: 1:1
relations (e.g. a record and its field) and 1:n relations (e.g. a List can
contain 0-n elements, a Maybe can contain 0-1 elements).
For 1:1 relations, the makeOneToOne function will let you build an accessor
by describing how to get the sub-element from the super-element, and how to map
a function over it. For instance, with a record:
recordFoo =
makeOneToOne
.foo
(\change record -> {record | foo = change record.foo})
recordBar =
makeOneToOne
.bar
(\change record -> {record | bar = change record.bar})1:n relations are more complex in terms of abstraction, but they are usually very easy to implement:
onEach =
makeOneToN
List.map
List.map
try =
makeOneToN
Maybe.map
Maybe.mapAccessors can be composed easily to describe relations:
myData = { foo = [ {bar = 3}
, {bar = 2}
, {bar = 0}
]
}
myAccessor = recordFoo << onEach << recordBarThen you use an action function to determine which kind of operation you want to do on your data using the accessor
getter = get myAccessor myData
-- returns [3, 2, 0]
setter = set myAccessor 2 myData
-- returns {foo = [{bar = 2}, {bar = 2}, {bar = 2}]}
transform = over myAccessor (\n -> n*2) myData
-- returns {foo = [{bar = 6}, {bar = 4}, {bar = 0}]}Applying an accessor on non-matching data structures will yield nice compile-time errors:
fail = (recordFoo << recordFoo) myData
--The 2nd argument to `get` is not what I expect:
--
--293| fail = get (recordFoo << recordFoo) myData
-- ^^^^^^
--This `myData` value is a:
--
-- { foo : List { bar : number } }
--
--But `get` needs the 2nd argument to be:
--
-- { foo : { a | foo : c } }Any accessor you make can be composed with any other accessor to match your new data structures:
myOtherData = {bar = Just [1, 3, 2]}
halfWay = try << onEach
myOtherAccessor = recordBar << halfWay
getter = get myOtherAccessor myOtherData
-- returns Just [1, 3, 2]Ellie default code with accessors.
build
elm make
run tests
elm-test
or
elm-test-rs
If you write new accessor combinators that rely on common library datas, I'll be happy to review and merge. Please include tests for your combinators.
