purescript - 如何显示 PureScript 列表中的每个项目

Question

纯脚本列表中是否有 foreach 方法？ foreach 方法获取列表的每一项并返回一个单位。

它是打印列表中每个项目的好方法。

编辑:我正在尝试下面建议的 traverse 方法，但出现错误

import Data.Traversable (traverse)
removeDuplicate :: AddressBook -> AddressBook 
removeDuplicate = nubBy (\a b -> a.firstName == b.firstName && a.lastName == b.lastName)    
let dedup = removeDuplicate addressBook  
traverse (\a -> log (showEntry a)) dedup

Compiling Main
Error found:
in module Main
at src/Main.purs line 73, column 3 - line 73, column 49

  Could not match type

    List Unit

  with type

    Unit


while trying to match type Eff
                             ( console :: CONSOLE
                             | t1
                             )
                             t2
  with type Eff
              ( console :: CONSOLE
              | e0
              )
              Unit
while checking that expression (discard (logShow ((...) addressBook))) (\__unused ->
                                                                          (discard (...)) (\__unused ->
                                                                                             ...
                                                                                          )
                                                                       )
  has type Eff
             ( console :: CONSOLE
             | e0
             )
             Unit
in value declaration main

where e0 is a rigid type variable
        bound at line 63, column 8 - line 78, column 38
      t2 is an unknown type
      t1 is an unknown type

See https://github.com/purescript/documentation/blob/master/errors/TypesDoNotUnify.md for more information,
or to contribute content related to this error.

Answer 1

您可以使用 for_ 它允许您对 Foldable 的每个项目执行“应用操作” ( List 和 Array 有 Foldable 实例)并忽略此操作结果:

module Main where

import Control.Monad.Eff.Console (logShow)
import Data.Foldable (for_)
import Prelude

main = do
  -- I'm using an Array here for simplicity, but the same goes for List
  let l = [1, 2, 3, 4]

  for_ l \i → logShow i

List还有 Show实例( second on the instances list )(还有数组 -
last on the instances list ) 以便您可以使用 logShow 打印它们如果它们包含类型为 Show 的值，则直接使用实例。