说我有一个看起来像这样的列表:
let identifiers = ["ABC123", "DEF456", "GHI789"]
我想知道元素“ DEF456”的索引。建议使用什么方法来完成此操作?
答案 0 :(得分:2)
标准库中findIndex
模块中的Base.List
函数可以满足您的要求。
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,大多数列表操作(包括findIndex
都是使用foldr
和foldl
来实现的。
-- 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))
答案 1 :(得分:2)
在daml 1.2
中,您可以像这样在elemIndex : Eq a => a -> [a] -> Optional Int
标准库模块中使用DA.List
函数:
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