Default values in records

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1 Question

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

2 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.

3 Source

• Haskell-Cafe on default values in a record structure

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