r3/SyntaxNotes
author Tony Garnock-Jones <tonygarnockjones@gmail.com>
Wed, 16 Jan 2019 17:15:58 +0000
changeset 438 1fe179d53161
parent 52 e8c6861a3e40
permissions -rw-r--r--
Add missing primitive implementation for the plain interpreter.

-*- outline -*-

* Basics, Revised Yet Again

The question is how to deal with currying, and reconciling the
difference between send syntax and pattern-match syntax. A chain of sends might
look like any of:

	r1 Class FieldNames
	r1 IfTrue: v1 IfFalse: v2
	dict Lookup: key [IfPresent: .v]: v IfAbsent: 123
	v1 == v2
	v1, v2, v3

A method clause:
  .m@[.self Lookup: .aKey]: ((self HasKey: aKey) IfTrue: (m IfPresent: (self At: aKey)) IfFalse: (m IfAbsent))

The other problem is centralised dispatch (by the interpreter and
current context) vs. distinguished-receiver ST80-style dispatch.

I guess it comes down to the shape of messages constructed by dispatch
syntax. With a central dispatcher, we get

	[0: [0: r1 1: Class] 1: FieldNames]
	[0: r1 IfTrue: v1 IfFalse: v2]
	[0: dict Lookup: key [IfPresent: .v]: v IfAbsent: 123]
	[0: [0: (==) 1: v1] 1: v2]
	[0: v1 1: v2 2: v3 Length: 3]

Corresponding patterns could be

	[.self Class]: ...
	[.self FieldNames]: ...

Hmm, essentially, when a pattern fails to match, it calls resend! So
if we had a high-priority pattern [.self Class FieldNames] matching
against [foo Class SomethingElse] we''d see the [foo Class] match the
[.self Class], resulting in [[_ FieldNames]: ... _: resend] or
similar. This, when sent [(itself) SomethingElse], would backtrack to
matching against some other receiver for [.self Class], if any.

	[.self IfTrue: .vt IfFalse: .vf]
	[.self Lookup: .aKey]

	"This next one is tricky:"
	m@[.self Lookup: .aKey [IfPresent: _]: _ IfAbsent: _]
	"Note in the above that the first _ is a value, ie. top!"
	"(rather than a pattern ie. bottom)"

	[.val == .other]
	[.v1, .v2, .v3]

Perhaps remove (,,) syntax? Maybe it could be sugar for list/stream
construction? For instance:

	(v1, v2, v3) ==> (v1 . (v2 . (v3 . Nil)))
	[.v1, .v2, .v3] ==> [.v1 . [.v2 . [.v3 . Nil]]]
			==> [(.) .v1 ((.) .v2 ((.) .v3 Nil))]
			decurrying ==>
		[0: (.) 1: ...? "difficulty."

Maybe keyword syntax instead:

	(v1, v2, v3) ==> (First: v1 Rest: (First: v2 Rest: (First: v3 Rest: Nil)))
	[v1, v2, v3] ==> [First: v1 Rest: [First: v2 Rest: [First: v3 Rest: Nil]]]
	[.v1, .v2, .v3] ==> [First: .v1 Rest: [First: .v2 Rest: [First: .v3 Rest: Nil]]]

That''s better. It makes sense, too, since once (,,) is no longer
tupling, the only n-ary syntax we have is the pattern/value
syntax. Binary operators turn into curried application, which is
tricky to pattern-match.

Shit, there's another tricky case: the difference between [x] and x;
or, more properly, between (x) and x.

	(x) ==> (First: x Rest: Nil)
	x   ==> x

Wrong! So, we need some kind of marker in the syntax. Unless... what
if we reinterpret (,) as a stream-cons operator rather than a
stream-separation operator?

(This, incidentally, is where python's (x,) syntax comes in.)

	(1, 2, 3) --> (First: 1 Rest: (First: 2 Rest: 3))
	(1, 2, 3, [])

Yuck. What about a right-associative flipped application operator (snoc)?

	1, 2, 3 <==> ((,) ((,) 3 2) 1)

Read them as begin?

	(begin x y z) ==
	(begin (begin x y) z) ==
	(begin x (begin y z)) ==
	(begin (begin x) (begin y) (begin z)) == etc.

	(x, y, z) ==
	((x, y), z) ==
	(x, (y, z)) ==
	((x), (y), (z)) == etc.

That's better. So, (,) becomes an n-ary tupling syntax again, in a
way.

	[x, y, z] ==
	[[x, y], z] == ... Hmm. Not so pretty here?

Actually, there's also a problem with scoping rules, if begin is used
like a let* or a letrec: What does ((.a <- b, c), a) mean?

* Basics

	pattern: value pattern: value ...	(fun (pattern value) ...)
	value, value, value, ...		(tuple value ...)
	value value				(adj value value)

	( value )				; grouping
	[ value ]				(quunquote value)
	#[ value ]				(quasiquote value)
	#( value )				(unquote value)
	. value					(quote value)

	atom					; symbols
	'another atom'				; symbols
	literal					; literal object sugar (strings, ints)
						; unit, nothing at all

	"a comment"				; comments

TODO: Bit syntax!

* Tuples, Records and Functions

[] is both the empty quoted tuple and the empty quoted function.

Tuples are sugar for functions with integer patterns! Like this:
(x, y, z)   <==>   (.length: 3 0: x 1: y 2: z)

(... or something. The ".length: 3" could instead be ".tuple: .tuple",
as an "interface marker" of some kind)

The empty tuple/function is "unit".

* Interpretation

Non-quoted tuples are sugar for monadic sequencing, that is /bind/
operations.

Non-quoted functions are messages sent to the ambient.

Non-quoted adjacency is function application == message send.

Non-quoted symbols are variable references.

Non-quoted literals are self-evaluating.

* Quoting

** Quote

** Quasiquote and unquote

** Quunquote