如何遍历PureScript List元素？是否有类似foreach的方法？调用traverse报错

Great question! Let's break this down step by step—covering the "foreach-like" methods you're looking for, plus why your traverse attempt might have failed.

1. Yes, PureScript has "foreach" equivalents for List #

If you want to iterate over a List, run a side effect (like printing) for each element, and return Unit (no useful result), you have a few solid options:

Option 1: for_ (most universal) #

for_ lives in Control.Monad and works with any Foldable type (not just List—think Array, Maybe, etc.). It's essentially a flipped version of traverse_, which makes the code read more naturally (like a "for each" loop).

Example usage for printing:

import Data.List (List, fromFoldable)
import Control.Monad (for_)
import Effect.Console (log)

main :: Effect Unit
main = do
  let numbers = fromFoldable [1, 2, 3] :: List Int
  for_ numbers \num -> log $ "Number: " <> show num

Option 2: forEach (List-specific) #

If you only need to work with List, there's a dedicated forEach function in Data.List. It does exactly what you'd expect: takes a List and a function that runs a side effect for each element.

Example:

import Data.List (List, fromFoldable, forEach)
import Effect.Console (log)

main :: Effect Unit
main = do
  let fruits = fromFoldable ["apple", "banana", "cherry"] :: List String
  forEach fruits \fruit -> log $ "Fruit: " <> fruit

2. Why your traverse attempt failed #

traverse is a powerful function, but it's not meant for side effects where you don't care about the result. Here's the key difference:

• traverse: Takes a function that returns an Applicative f a, runs it over each element, and collects all the results into an f (List a). So if you use traverse log myList, you'd get an Effect (List Unit)—a computation that returns a List of Unit values.
• traverse_ (note the trailing underscore): Does the same iteration and side effects, but discards all results, returning just f Unit (which is exactly what you want for printing).

If you tried using traverse directly, you probably got a type error because your code expected an Effect Unit but got an Effect (List Unit) instead. The fix is simple: swap traverse for traverse_ (or use for_ as we saw earlier).

Example of the corrected traverse_ usage:

import Data.List (List, fromFoldable)
import Control.Monad (traverse_)
import Effect.Console (log)

main :: Effect Unit
main = do
  let items = fromFoldable [True, False, True] :: List Boolean
  traverse_ (\item -> log $ "Boolean: " <> show item) items

Quick Recap #

• Use for_ for a universal, readable "foreach" style loop (works with all Foldables)
• Use forEach if you only need to target List specifically
• Avoid traverse for side-effect-only iteration—use traverse_ instead to avoid type errors and unnecessary result collection

内容的提问来源于stack exchange，提问作者Knows Not Much