Function builders

[Chris_Lattner3 (Chris Lattner)]

@John_McCall

I've posted a draft proposal which provides a lot of detail about this feature, but as a quick summary, the basic idea is that we take the "ignored" expression results of a block of statements — including in nested positions like the bodies of if and switch statements — and build them into a single result value that becomes the return value of the current function.

Hi John,

This is a super interesting proposal. I am so happy to see a direction that takes eDSL's seriously - this is a very exciting opportunity for Swift, one that could solve a huge range of issues and problems. It could even potentially get us to the golden world where all "statements" are just standard library features with a default statement rewrite rule in the standard prelude / stdlib.

Stepping back from that beautiful world, the quote above highlights an interesting point. I'm still trying to understand the feature itself (is there a patch implementing the feature available somewhere?), but it seems like there are two things going on here:

1. You're providing a very general statement rewriting facility, where a syntactic transformation is changing the statement AST into a series of function calls.

2. You're transforming if/switch "into a single result value that becomes the return value of the current function"

Have you considered decomposing these into two separate and orthogonal -- but composable features? One that allows if/switch to return values (this is something that has been highly requested for a long time) and one that allows "function builders" to transform the AST into overloadable function calls (e.g. along the lines of tf autograph)?

The composition of these two features seem like they'd be far more powerful and also simpler than the feature as proposed. I'm very interested in allowing this to eventually feature creep towards defining CUDA kernels and other accelerator functions for machine learning applications right within Swift (i.e., true DSLs, not just a thing that rewrites ignored top level expressions).

-Chris