Greetings,

I'm struggling to find a way to define an "Alist" once, and then simply add-in "Fields" to it incrementally.. without having to keep using new identifiers/variables to hold the result of each "addin" expression.

I understand that pure Functional Programming doesn't use destructive state changes, but I'm wondering if there is a way to make this happen using Monads, for example (which I have a cursory understanding of..)

Instead of doing this:

type Root = String

type Oct = Integer

type Mode = Integer

data Field = Root Root | Oct Oct | Mode Mode deriving (Show)

type Alist = [Field]

addin :: Alist -> Field -> Alist

addin p f = f:p

p0 :: Alist

p0 = []

p1 = addin p0 (Root "c")

p2 = addin p1 (Oct 4)

p3 = addin p2 (Mode 3)

p4 = addin p3 (Oct 3)

-- ... p42, etc

For the Alist version, when adding-in a Field with the same constructor-name as one that has already been added-in, it adds-in a brand new entry (with a repeated key-name (in the form of another constructor) and its associated value), as a normal association list does..

I'm also looking to define a Property-List version, where a Field-name/constructor can update a previous value (if the Field has already been added), or add-in a new entry if the Field hasn't been added yet.

Many Thanks,

Tom Jordan