Haskell Repa - select函数有点混乱

Question

我对repa包中的select函数有点困惑：

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

给出结果

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

我以为我在0和10之间，或者0和9之间，为什么是在0和8之间？

repa 2.0.2.1

Answer 1

看起来它生成了一个长度为len - 1的数组，在本例中长度为9。它给出了0-8范围内的索引。我同意文档可以更清晰。

如果你看一下源代码，就会发现select是用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，给定n的“索引范围”包括所有的索引x，使得0 <= x < (n-1)

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