Default values in records

Question

Is there a way to declare default value of some fields when defining a new record type?

Answer

You cannot associate default values with a type, but you can define as many record values as you like and use them as basis for updating elements.

data Foo = Foo { bar :: Int, baz :: Int, quux :: Int }

fooDefault = Foo { bar = 1, baz = 2, quux = 3 }

newRecord = fooDefault { quux = 42 }

If you only want some of the fields to be defaulted, you can make them undefined in the default record. Unfortunately this won't be statically checked by the compiler.

Source

• Haskell-Cafe on default values in a record structure