數組的模式匹配

Question

Haskell在Data.Array.IArray提供了不可Data.Array.IArray的類型Array 。 這允許在功能代碼內部使用數組。 是否可以將Array與模式匹配，就像列表一樣？

我想下面的代碼工作...

function :: Ix a => Array a b -> ...

function [| x |]         = ... -- matches if arg has one element
function [| x, y |]      = ... -- matches if arg has two elements
function [| x, ..., y |] = ... -- matches if arg has more than 3 elements and
                               -- binds the first and last element with x and y

注意：此功能以Rust語言存在，請參閱https://doc.rust-lang.org/book/slice-patterns.html

Answer 1

首先，請注意，Haskell數組可能是多維的。

您可以這樣編寫示例函數：

import Data.Array.IArray
import Data.Ix

foo arr
  | first == last = print x      -- single element case
  | otherwise     = print (x,y)  -- more than one element case
  where (first,last) = bounds arr
        x = arr ! first
        y = arr ! last

Answer 2

我將針對Data.Vector或Data.Vector.Unboxed回答這個問題，因為我認為對於多維Data.Array.IArray而言，這個問題真的沒有Data.Array.IArray ...

您可以在Data.Sequence中執行類似於ViewL和ViewR的Data.Sequence 。 適應這一點，您可以

data ViewL a
    = EmptyL        -- ^ empty vector
    | a :< Vector a -- ^ leftmost element and the rest of the vector

data ViewR a
    = EmptyR        -- ^ empty vector
    | Vector a :> a -- ^ rest of the vector and the rightmost element

然后，您可以定義函數

viewl :: Vector a -> ViewL a
viewl v | null v = EmptyL
        | otherwise = head v :< tail v 

viewr :: Vector a -> ViewR a
viewr v | null v = EmptyR
        | otherwise = init v :> last v 

viewlr :: Vector a -> (ViewL a, ViewR a)
viewlr v = (viewl v, viewr v) -- or `viewl &&& viewr` if you like Control.Arrows

並將它們與ViewPatterns擴展一起使用：

function :: Vector a -> ...
function (viewl -> x :< EmptyL)      = ... -- matches if arg has one element
function (viewl -> x :< y:< EmptyL)  = ... -- matches if arg has two elements
function (viewlr -> (x :< _, _ :> y) = ... -- matches `x` and `y` to the first and
                                           -- last elements
function (viewlr -> (x :< _ :< _ :< _, _ :> y) = ... -- matches if arg has more
                                                     -- than 3 elements and binds 
                                                     -- the first and last element
                                                     -- with x and y

由於tail和init是Data.Vector O（1）切片，因此懶惰地僅根據需要查看向量中的所有元素。