PureScript - NewType的访问属性

Question

考虑下面的代码示例，它创建了一个新类型来表示客户模型：

module Main where

import Effect (Effect)
import Effect.Console ( logShow )
import Prelude (Unit,(+),(/),(*),(-), (<>),discard)

newtype Customer 
  = Customer
  { firstname :: String
  }

sample :: Customer
sample = Customer
  { firstname : "Average"
  }

first :: Customer -> String
first a = _.firstname a

main = do
  logShow ( first sample )

预期的输出将是值Average，它等于sample.name，但是会产生一个错误：

 Could not match type
  
    { firstname :: t0
    | t1
    }
  
  with type
  
    Customer
  

while checking that type Customer
  is at least as general as type { firstname :: t0
                                 | t1
                                 }
while checking that expression a
  has type { firstname :: t0
           | t1
           }
in value declaration first

where t0 is an unknown type
      t1 is an unknown type

这是一个很好的错误，但并没有解释如何实际访问这个值。

如何访问作为newType创建的对象的值？

Answer 1

你必须做

first :: Customer -> String
first (Customer a) = _.firstname a

因为newtype确实是一种新的类型。

Answer 2

另一种方法是派生特定newtype的newtype实例，该实例公开某些函数，使您可以处理由Newtype包装的数据。

derive instance newtypeCustomer :: Newtype Customer _
 
first :: Customer -> String
first  = (_.firstname <<< unwrap)