elmMail1.hs

-- Given these simplified types:
data Model = Model { something :: SomeSubData }

data SomeSubData = SomeSubData { moreThing :: Integer }

-- And given this value
where
  model = Model (SomeSubData 42)


-- You could do the way that Elm does now (although elm has the existing record inside the {} and haskell has it outside, so this is how haskell does it):
newModel = model {
  something = (something model) {
    moreThing = moreThing (something model) + 1
    }
  }


-- The above set newModel as { something = { moreThing = 43 } } in Elmish parlance, and is very similar to how it is done in Elm itself, however Haskell has HKT's, which allow for a variety of other methods such as:


-- references+lenses (not possible to do anything even remotely like this in Elm right now due to not being able to operate over types, but it is the most succinct):
$(mkLabels [ 'Model, 'SomeSubData ]) -- Only need to do this once

newModel = modify (+1) (moreThing . something) model


-- SYB (A generic library that handles tasks like this with is, it comes bundled with Haskell
incMoreThing v s@(SomeSubData moreThing) = s { moreThing = moreThing + v } -- Per change you want to make anywhere in the record path

newModel = everywhere (mkT (incMoreThing 1))


-- Semantic Editor Combinators (low-level version of SYB, no libraries needed, and you could make these 'now' in Elm, and I have to an extent, but it is very easy in haskell to define it from scratch, as doing here, still very wordy as it requires one function made per key):
type SemEdit a = a -> a
type Lifter p q = SemEdit p -> SemEdit q

onSomething :: Lifter SomeSubData Model
onSomething f (Model something) = Model (f something)

onMoreThing :: Lifter Integer SomeSubData
onMoreThing f (SomeSubData moreThing) = SomeSubData

newModel = (onSomething . onMoreThing) (+1) model