Define g ⨟ f = f ∘ g

[tkf]

This PR adds a simple definition ⨟(fs...) = ∘(reverse(fs)...) to the new operator added in #34722. This is useful for abstracting out pipelined operations like x |> f |> g |> h as f ⨟ g ⨟ h. For example:

mapping(f) = xs -> Base.Generator(f, xs)
filtering(f) = xs -> Iterators.filter(f, xs)
partitioning(n) = xs -> Iterators.partition(xs, n)
using Base.Iterators: flatten

# Iterator-to-iterator transformation to do some normalization and filtering
# with sliding window (a toy example):
ixf = partitioning(10) ⨟
    mapping(x -> (x , mean(x))) ⨟
    filtering(((_, m),) -> m > 0) ⨟
    mapping(((x, m),) -> x ./ m) ⨟
    flatten

collect(ixf(randn(100)))

cc @jw3126 @schlichtanders @ararslan

[ararslan]

I don't know what the symbol is

[tkf]

In my browser:

[ararslan]

Char(0x2a1f) doesn't render in my terminal either (same system: Ubuntu derivative). It'd be difficult for me to believe I'd be the only one with this issue, so it's difficult to support adding something I can only read in other people's screenshots. 😛

[tkf]

I just learnt from #34835 (comment) that fileformat.info shows a list of fonts that supports a given unicode (within the fonts installed in their server). Maybe useful as sampling points:

• U+2A1F ⨟ https://www.fileformat.info/info/unicode/char/2a1f/fontsupport.htm
• U+2218 ∘ https://www.fileformat.info/info/unicode/char/2218/fontsupport.htm

So yes, U+2218 definitely is supported by more fonts than U+2A1F.

FYI here is the list of non-ASCII names exported from Base:

julia> [x for x in (string(x) for x in names(Base)) if !isascii(x)]
26-element Array{String,1}:
 "÷"
 "π"
 "ℯ"
 "∈"
 "∉"
 "∋"
 "∌"
 "∘"
 "√"
 "∛"
 "∩"
 "∪"
 "≈"
 "≉"
 "≠"
 "≡"
 "≢"
 "≤"
 "≥"
 "⊆"
 "⊇"
 "⊈"
 "⊉"
 "⊊"
 "⊋"
 "⊻"

There are some variations in font support for these unicode characters. For example, U+212F ℯ is supported less than U+2218 ∘ but more than U+2A1F ⨟:

• U+212F ℯ https://www.fileformat.info/info/unicode/char/212f/fontsupport.htm

[laborg]

I can confirm that this is not shown on a vanilla ubuntu + firefox setup. IMHO the short list of supported fonts is good to estimator for the amount of confusion to expect from adding this.

[Keno]

Somebody should file a request with whatever authority is in charge of font authoring for that combination ;).

[andyferris]

I agree an operator with this semantic would be great; unfortunately, I also don't see this rendered correctly (in my browser or terminal).

[jlapeyre]

Maybe it's an ubuntu quirk. It's rendered in my browsers and terminals. I'm running Arch, which doesn't give you anything out of the box.

[JeffBezanson]

I knew you were up to something when you added this to the parser :)

Works fine for me on two ubuntu 18.04 systems. I hesitate to add an operator with potentially poor font coverage to Base though.

[jw3126]

Works for me on ubuntu 18.04 + firefox

[tkf]

Of courses, we can solve the font problem by defining something with ASCII

[schlichtanders]

font is correctly displayed for me on mac catalina 10.15.2 with all three, chrome, firefox and safari, with default font settings