daml - 如何在 DAML 的列表中找到特定元素的索引？

Question

假设我有一个如下所示的列表：

let identifiers = ["ABC123", "DEF456", "GHI789"]

我想知道元素“DEF456”的索引。完成此任务的推荐方法是什么？

Answer 1

findIndex标准库中模块中的Base.List函数，做你想做的事。

daml 1.0 module FindIndex where

import Base.List
import Base.Maybe

test foo : Scenario {} = scenario
  let
    identifiers = ["ABC123", "DEF456", "GHI789"]
    index: Maybe Integer = findIndex ((==) "DEF456") identifiers

  assert $ index == Just 1

在 DAML 中的大多数列表操作，包括findIndex使用foldr和实现foldl。

-- Returns the index of the first element in the list satisfying the predicate, or M.Nothing if there is no such element.
def findIndex (f: a -> Bool) (xs: List a) : Maybe Integer =
  headMay (findIndices f xs)

-- Returns the indices of all elements satisfying the predicate, in ascending order.
def findIndices (f: a -> Bool) (xs: List a) =
  let work acc x =
        let i = fst acc
        let is = snd acc
        tuple (i + 1) (if f x then cons i is else is)
  reverse (snd (foldl work (tuple 0 nil) xs))

Answer 2

daml 1.2您可以像这样使用标准库模块中的elemIndex : Eq a => a -> [a] -> Optional Int函数：DA.List

daml 1.2 module MyModule where

import DA.List

indexOfElement = scenario do
  let identifiers = ["ABC123", "DEF456", "GHI789"]
      index : Optional Int = elemIndex "DEF456" identifiers
  assert $ index == Some 1
  return index