haskell,design,functional-programming,immutability,purely-functional , Mutable vs Immutable: how to implement editing user profile in functional\pure way?


Mutable vs Immutable: how to implement editing user profile in functional\pure way?

Question:

Tag: haskell,design,functional-programming,immutability,purely-functional

I'm learning functional programming and try to apply its ideas in day job with imperative languages. My favourite idea is data immutability. I can apply this concept in my programs and try to make all my data structures and model objects immutable. But I have a standard feature in my app: edit user profile. So, I got profile data from the server and create an immutable domain model of it: class UserProfile and show its info to the user. User can edit any fields: change name, surname, age, interests, whatever. The first idea is to make UserProfile global, mutable and create a setter for each field. But I want to handle this logic in a pure functional way.

So, what are the possible ways of handling this logic correctly in a functional way?


Answer:

You might find the Updatable type from mvc-updates useful. The basic idea is that you build an Updatable value by supplying a concurrent input (i.e. a Controller) and a Fold over that input:

on :: Fold e a -> Managed (Controller e) -> Updatable a

Here are two example inputs:

lastLine :: Updatable (Maybe String)
lastLine = on last stdinLines

seconds  :: Updatable Int
seconds  = on length (tick 1.0)

Then you can combine them into larger Updatable values using Applicative operations:

data Example = Example (Maybe String) Int deriving (Show)

example :: Updatable Example
example = Example <$> lastLine <*> seconds

That produces an Updatable record that updates whenever one of its fields updates.

Finally, you can attach a listener to an Updatable value using listen:

listen :: (a -> IO ()) -> Updatable a -> Updatable a

... and you can set things in motion using runUpdatable:

runUpdatable :: Updatable a -> IO ()

So the full example would be:

main = runUpdatable (listen print example)

... and it will behave like this:

>>> main
example : Example Nothing 0
Test<Enter>
example : Example (Just "Test") 0
example : Example (Just "Test") 1
example : Example (Just "Test") 2
ABC<Enter>
lastLine: Just "ABC"
example : Example (Just "ABC") 2
example : Example (Just "ABC") 3

Every time you type a new line of input the left field updates and every time a second passes the right field updates. Each update triggers the listener, which in this case is just print.

You can learn more by reading the mvc-updates documentation.


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