arrays - Haskell Repa --- 选择函数有点混乱

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 到 10 或 0 到 9 之间。为什么它在 0 到 8 之间？

修复 2.0.2.1

Answer 1

Looks like it produces an array of length len - 1, which is 9 in your case. Which gives you indices in the [0-8] range. I agree that the documentation could be more clear.

If you look at the source, select is implemented in terms of selectChunkedP:

-- | 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.

Apparently, 'extent of indices' for a given n includes all indices x such that 0 <= x < (n-1):

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