Support tail recursion

[tangentstorm]

Tail recursion can be detected when

• an explicit definition is running
• the result of the current sentence is guaranteed to be the result of the verb
• the sentence issues a recursion to the same explicit definition
• all tokens have been moved from the queue to the stack in the parser
• no names are on the stack in the parser

-- @HenryHRich 13:15, 12 June 2017 (UTC)

[moon-chilled]

I think the most interesting part of tail recursion/tail calls is not actually tearing down the stack frame early, but rather decrementing the reference counts on local variables early, in order to reduce space consumption and enable more in-placing. And decrefing can happen much earlier than the final call in a definition. E.G. this should be able to work in place:

f=. {{ 1 (2}) y }}
g=. {{
 a=. i.y
 b=. f a
 b }}
g 1e8

[moon-chilled]

(also: of course should be togglable, though probably orthogonal to debug mode, as you might want to include debugging without giving up performance)

[HenryHRich]

This should be done by explicit user action. Perhaps a keyword recur. which is 'goto_0' but clearing local variables and setting new x/y.