include pattern matching forms in more binding positions

[spdegabrielle]

include pattern matching forms in more binding positions (e.g. define, let/for clauses) -
@samth 2 Oct 2012

[AlexKnauth]

Cross-reference with point (4) of #23, since the syntax of patterns affects whether they can be used unambiguously in things like optional-arguments with defaults in define, as @dedbox pointed out

[rocketnia]

If syntax-parse is merged with matching and matching is used for argument lists, then I think the syntax would change in a way that's something like this:

; before
(define (my-print [out (current-output-port)])
  ...)

; after
(define (my-print {~optional out #:defaults ([out (current-output-port)])})
  ...)

; after defining a `define-splicing-syntax-class` helper
; (or maybe it would be `define-splicing-class`)
(define (my-print {~opt out (current-output-port)})
  ...)

The ambiguity would have come from the original syntax's use of parens without a prefix operator inside them. The revision just adds ~opt to keep it clear why the parens are there.

[dedbox]

While I agree a pattern-friendly syntax for args with defaults would be good, I think making match patterns appear more like syntax patterns would be a mistake unless we also intend to unify the semantics (and consequent developer experience) of expansion and evaluation. See #23 for my rationale.