Create new record with change in one value

Question

Suppose my record (with large number of fields) is defined like this:

data Sample_Record = Sample_Record { record_field1 :: Int,
                                     record_field2 :: Int,
                                     record_field3 :: Float
                                    }

a = Sample_Record { record_field1 = 4,
                    record_field2 = 5,
                    record_field3 = 5.4
                  }

Can I make a new record of the type Sample_Record from a which has one of it's field modified ?

Answer 1

Yes. We have a lot of ways. The simple is

foo b = b {record_field1 = 1}

> foo a
 Sample_Record { record_field1 = 1,record_field2 = 5, record_field3 = 5.4 }

We have some extensions. WildCards allow not to use all record in pattern,

{-# LANGUAGE RecordWildCards #-}
bar b@(Sample_Record {record_field1 = 1,..}) = b {record_field1 = 10}
bar b@(Sample_Record {record_field1 = 2,..}) = b {record_field1 = 20}

With NamedFieldPuns extension we could use record fields as values without extra boilerplate (f (C {a=a}) = a is same as f (C {a}) = a)

{-# LANGUAGE NamedFieldPuns #-}
baz b@(Sample_Record {record_field1, record_field2, record_field3 = 0}) = 
   b{ record_field3 = record_field1 + record_field2 }