field-mapping.md

field mapping

Introducing syntax for changing one specific field value

!Name :
    -> (-> value -> valueMapped)
    -> ; record ; #Name value
    -> ; record ; #Name valueMapped

which one previously had to write as

(\alter -> \record -> { record | name = record.name |> alter })

• verbose
• really hard to scale (nested lambdas, branching altering different fields, indentation 🚀)
• possibly confusing for beginners (what does | mean? why are consecutive fields updated with ,, nested updates look awful, ...)

model
    |> !PlayerPosition (!Y (\_ 0))
    |> !PlayerVelocity
        (\velocity
            velocity
                |> !X (\* 0.95)
                |> !Y (\* 0.95)
                |> !Y (\- 1)
        )
    |> checkForCollision

Apart from syntax, the proposed syntax also allows changing the field value's type

--  or ()
Blank =
    #Blank

succeed (; #Name #Blank ; #Status #Blank ; #Metadata metadataDefault)
    |> field !Name "name" string
    |> field !Status "status" string

• simple
• declarative succeed showing the final shape
• field value changes are drastically simpler and less verbose
• could be misused to alter previous fields

To discuss

• symbol
  • vs !
  • vs /
  • !, / look like "action"
  • ! looks like . (field access) with something else on top
  • subjective / is hard and confusing to read with lambdas
• representation
  • vs function
  • vs infix
  • infix is slightly more compact
  • function reads better alongside other transformations
  • infix can't be curried
  • all infixes are just symbols, !<Name> would be confusing
  • → a function solves the problems well enough to not need a more complex system around it

extras

if 1-field records and 1-variant choices unify

Pet specificProperties =
    ; specificProperties
    ; #Name String
    ; #Hunger Progress

Cat =
    #Cat (#NapsPerDay Float)

Dog =
    #Dog (#BarksPerDay Float)

sit : -> Dog -> Dog
sit =
    !Dog (!Hunger (Progress.by 0.01))

howdy =
    #Cat (; #Name "Howdy" ; #Hunger Progress.begin ; #NapsPerDay 2.2)

howdy |> sit -- error

removing the intermediate need for boilerplate.

field mapping composition

module Dog exposing (hungerAlter)

Dog =
    #Dog (; #BarksPerDay Float ; #Hunger Progress)

hungerAlter : -> (Progress -> Progress) -> (Dog -> Dog)
hungerAlter =
    !Dog << !Hunger

Above might be prone for abuse to over-nest your structures and rely on primitives too often.

It might be best to add a few paragraphs on the dangers of the pattern.