Haskell Repa --- select函数有些令人困惑

Question

我对repa包中的select函数有些困惑：

select (\i -> True) (\i -> i) 10

提供结果

[0,1,2,3,4,5,6,7,8]

我原本认为它应该在0到10或0到9之间。为什么它只在0到8之间呢？

repa 2.0.2.1

- Yrogirg

Answer 1

看起来它生成一个长度为len - 1的数组，在您的情况下为9。这为您提供了[0-8]范围内的索引。我认为文档可以更清晰。

如果您查看源代码，select是基于selectChunkedP实现的：

-- | Select indices matching a predicate, in parallel.
--   The array is chunked up, with one chunk being given to each thread.
--   The number of elements in the result array depends on how many threads 
--   you're running the program with.
selectChunkedP 
    :: forall a
    .  Unbox a
    => (Int -> Bool)    -- ^ See if this predicate matches.
    -> (Int -> a)       --   .. and apply fn to the matching index
    -> Int              -- Extent of indices to apply to predicate.

显然，对于给定的n，“指标范围”包括所有满足0 <= x < (n-1)的指标x：

Prelude Data.Array.Repa> extent $ select (const True) id 10
Z :. 9

- Mikhail Glushenkov