Shorthand way for assigning a single field in a re

Question

Let's say I have the following record ADT:

data Foo = Bar { a :: Integer, b :: String, c :: String }

I want a function that takes a record and returns a record (of the same type) where all but one of the fields have identical values to the one passed as argument, like so:

walkDuck x = Bar { a = a x, b = b x, c = lemonadeStand (a x) (b x) }

The above works, but for a record with more fields (say 10), creating a such function would entail a lot of typing that I feel is quite unnecessary.

Are there any less tedious ways of doing the same?

Answer 1

Yes, there's a nice way of updating record fields. In GHCi you can do --

> data Foo = Foo { a :: Int, b :: Int, c :: String }  -- define a Foo
> let foo = Foo { a = 1, b = 2, c = "Hello" }         -- create a Foo
> let updateFoo x = x { c = "Goodbye" }               -- function to update Foos
> updateFoo foo                                       -- update the Foo
Foo {a = 1, b = 2, c = "Goodbye" }

Answer 2

This is a good job for lenses:

data Foo = Foo { a :: Int, b :: Int , c :: String }

test = Foo 1 2 "Hello"

Then:

setL c "Goodbye" test

would update field 'c' of 'test' to your string.

Answer 3

You don’t need to define auxiliary functions or employ lenses. Standard Haskell has already what you need. Let’s take the example by Don Stewart:

data Foo = Foo { a :: Int, b :: Int , c :: String }

test = Foo 1 2 "Hello"

Then you can just say test { c = "Goodbye" } to get an updated record.