通过更改一个值创建新记录

Question

假设我的记录(包含大量字段)定义如下:

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
                  }

我可以创建一个新的记录类型Sample_Record,a其中一个字段被修改？

Answer 1

是.我们有很多方法.简单就是

foo b = b {record_field1 = 1}

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

我们有一些扩展.WildCards允许不使用模式中的所有记录,

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

通过NamedFieldPuns扩展,我们可以使用记录字段作为值而无需额外的样板(与之f (C {a=a}) = a相同f (C {a}) = a)

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