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0 pointszfnmxt2y ago0 comments

> No, that would error in julia.

Okay, but '[[1,2,3] [4,5,6]] .+ 4` does work! Looks like Julia distinguishes between a two-dimensional array and an array of arrays (which Futhark does not). To me, it feels like semantically '[[1,2,3] [4,5,6]] .+ 4` is two levels deep even if operationally it has some underlying flat representation. (But I'm not familiar with Julia's programming model.)

> The trick is that it just turns into function calls.

Ah, okay. I think `a .+ f.(b)` can roughly be understood in Futhark as `map^N (+) a (rep^M (map^Q f (rep^S b)))` where `map^N` means do N `map`s and you can locally determine N, M, Q, S from the input types. We actually essentially had this in our very first prototype of AUTOMAP but found it insufficient for our purposes because it doesn't play nice with arguments whose types aren't fully known (i.e., that have type variables) and we cannot wait until runtime to figure out those types.

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eigenspace2y ago

> Okay, but '[[1,2,3] [4,5,6]] .+ 4` does work! Looks like Julia distinguishes between a two-dimensional array and an array of arrays

Ah, yes, Julia distinguishes between multidimensional arrays, and arrays of arrays.

The thing you wrote is actually syntax sugar for “horizontal concatenation” (hcat), I.e. when one writes `[A B]` that makes a flat matrix. So what you wrote is actually just (mildly confusing) syntax sugar for the matrix

    [1 4
     2 5
     3 6]

which is an array of scalars in julia, not an array of arrays. If you had written `[[1,2,3], [4,5,6]] .+ 4` you’d have an array of arrays, and hence an error.

Given that Furthark chose to have multidimensional arrays being semantically arrays-of-arrays, I see why it’s important for you to automatically descend recursively through multiple levels.

I still think it’s maybe a bit of a footgun that users cant choose at the call-site if their function is broadcasted or not though (that’s the really nice thing about julia’s dot-broadcasting for me anyways).

adgjlsfhk12y ago

the part that is confusing you here is actually just [[1,2,3] [4,5,6]]. this is syntax for constructing a matrix (as opposed to [[1,2,3], [4,5,6]] with a comma which makes a vector of vectors). Julia has first class support for n dimensional arrays which is very useful for scientific/mathy code

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