InlinePart. What is it and what happened to it?

Question

So a while back I was searching for an unused high-application order symbol I could use for an OOP framework and I ran across InlinePart (esc @> esc) in my UnicodeCharacters.tr file with an application precedence of 763 (e.g. when parsing an expression using this its formatting rules are called before those of, say, []).

When I actually use the symbol though I get this:

In[63]:= a \[InlinePart] b[10]

Out[63]= RowBox[{a, InlinePart, b}][10]

For those curious the symbol looks like this:

And the only reference I could find to it (outside of styling stuff or other formatting things) was this.

So what was it? Why'd it go?

Answer 1

InlinePart or @> was an operator that was introduced in one of the pre-release/betas for version 10 and subsequently removed before public release. It was briefly available publicly in the Raspberry Pi version of Wolfram Language that co-existed with the pre-releases at the time.

The operator allowed you to access elements of a list with an infix operator. Both of these would have been equivalent:

list = {1, 2, 3};

list @> 2 (* InlinePart *)
list[[2]] (* Part *)

I think it was also more flexible than that and allowed you to access keys of Associations with assoc @> "key" syntax. The thinking probably might have been that this syntactic sugar would make it easy to chain or "drill" into a dataset with dataset @> key1 @> key2 @> key3

If I had to guess why it was discontinued, I would think that it was because it didn't bring much to the table while adding a lot of overhead (both for the end user and for the developers) and fundamentally altering a core symbol Part.

• You couldn't use ;; or Span for flexible indexing.
• There is the potential for visual confusion with @ and friends.
• I don't remember, but there might have been some precedence issues as well. For example, f @ g @> 2 might have been parsed as (f @ g) @> 2 instead of f @ (g @> 2) which is different from using [[]] (I could be misremembering this).

I think in lieu of this operator, Associations was modified to support the function call syntax so that you could do assoc @ "key" which achieved the same while being more idiomatic/Mathematica like.

My memory is hazy and some parts about the motivations are speculative, but I think this is a fair summary of what this operator did during its short life. Taliesin Beynon will know more about this if you're really interested in an authoritative answer :)