I would like to extend upon Mark's suggestion of .field= functions that
parallel the .field functions.
I find I want these as a pair more often than a setter functions on its own.
Lets use @field as an example syntax.
@field :
{ get : { a | field : b } -> b
, set : b -> { a | field : b } -> { a | field : b }
}
@field =
{ get = \r -> r.field
, set = \v r -> { r | field = v }
}
This is in the spirit of arturopala/elm-monocle's Lens
<http://package.elm-lang.org/packages/arturopala/elm-monocle/1.4.0/Monocle-Lens>
type, whose example:
streetNameOfAddress : Lens Address String
streetNameOfAddress =
let
get a = a.streetName
set sn a = { a | streetName = sn }
in
Lens get set
would simplify to:
streetNameOfAddress : Lens Address String
streetNameOfAddress =
Lens @streetName
I am using a similar lens type as part of an update composition library
that allows simplification of:
update msg model =
case msg of
SearchPage msg_ ->
let
(searchPage_, cmd) = SearchPage.update msg_ model.searchPage
in
( { model | searchPage = searchPage_ }, cmd )
into:
update msg model =
case msg of
SearchPage msg_ ->
Compose.update msg_ model @searchPage SearchPage.update
The above example of Compose.update is highly simplified, it can do much
more than just reduce boilerplate code. I have reduced the example to
highlight to focus on the @field syntax.
Compose.update can only reduce boilerplate code if @searchPage does not
have to be expressed as:
{ get = .searchPage, set = \v r -> { r | searchPage = v } }
To this end I have a pre-elm-compile script that looks for expressions of
the form At.field and builds an elm source file of the form:
module At exposing (field, name, searchPage)
field = { get = .field, set = \v r -> { r | field = v } }
name = { get = .name, set = \v r -> { r | name = v } }
searchPage = { get = .searchPage, set = \v r -> { r | searchPage = v } }
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