Shorthand way for assigning a single field in a record, while copying the rest of the fields?

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

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

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.

Answer 3

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.