Shorthand way for assigning a single field in a record, while copying the rest of the fields?
HaskellRecordHaskell Problem Overview
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?
Haskell Solutions
Solution 1 - Haskell
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" }
Solution 2 - Haskell
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.
Solution 3 - Haskell
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.