1	# rule tail: a def whose first live parameter is a List or a String calls
2	# itself where the call is not the whole statement: `h <> go(t)`,
3	# `(1 + go(t) : U32)`, `+rest = go(t)`. The test is on that parameter's type
4	# alone, never on whether the self-call shrinks it: a def that recurses on
5	# a later Nat is reported too, and one whose list comes second is not. Each
6	# such call holds a frame until the rest of the input is done, and the JS lane
7	# overflows its stack at a few thousand to ~64K elements (bend-http on a 48KB
8	# header, agora at ~4,900 entries); native is fine. Carry an accumulator and
9	# make the self-call the whole statement (reverse once at the end if order
10	# matters). Idiomatic code does this on purpose, so the rule is noisy: off
11	# unless asked. Everything inside a `Bool.pick(..)` is skipped, whether or
12	# not `pick` reports it; laws and proofs are exempt, and a def with no type
13	# at all fills a law, so it is a proof.
14	import Base
15	import ../../src.bend as Src
16	import ../../finding.bend as F
17	import ../../syntax/lex.bend as Lex
18	import ../../syntax/tree.bend as Tree
19	import ../../lazy/lazy.bend as Lazy
20	import ../calls.bend as Calls
21	
22	# is a statement's chain exactly a call of the name, `name(..)`?
23	def bare(kids: Tree.Node, +name: String) -> Bool:
24	  match kids:
25	    case Tree.NCons{Tree.Leaf{Lex.Tok{k, +t, l, c}}, Tree.NCons{g, Tree.NNil{}}}:
26	      Bool.and(String.eq(t, name), Tree.opens(g, "("))
27	    case other:
28	      False{}
29	
30	# the self-calls not in tail position; skip: the chain is a bare self-call
31	def walk(nn: Tree.Node, +name: String, +path: String, skip: Bool) -> List<&2, F.Finding>:
32	  match nn:
33	    case Tree.NCons{Tree.Leaf{Lex.Tok{k, +t, +l, +c}}, Tree.NCons{Tree.Group{Lex.Tok{_, +o, _, _}, +kids, _}, rest}}:
34	      +call = String.eq(o, "(")
35	      +me = Bool.and(Bool.and(call, String.eq(t, name)), Bool.not(skip))
36	      +inside = walk(kids, name, path, False{})
37	      +more = List.concat(&2, F.Finding, [
38	        Bool.pick(List<&2, F.Finding>, Bool.and(call, String.eq(t, "Bool.pick")), [], inside),
39	        walk(rest, name, path, False{})])
40	      Bool.pick(List<&2, F.Finding>, me,
41	        F.Finding{path, l, c, U32.from_nat(String.length(name)), "tail",
42	          "This call to " ++ name ++ " is not a tail call, so a long list or string overflows the stack on the JS lane; carry an accumulator."}
43	          <> more,
44	        more)
45	    case Tree.NCons{Tree.Group{open, kids, close}, rest}:
46	      List.concat(&2, F.Finding, [walk(kids, name, path, False{}), walk(rest, name, path, False{})])
47	    case Tree.NCons{Tree.Stmt{kind, +kids, body}, rest}:
48	      List.concat(&2, F.Finding, [walk(kids, name, path, bare(kids, name)), walk(body, name, path, False{}),
49	        walk(rest, name, path, False{})])
50	    case Tree.NCons{h, rest}:
51	      walk(rest, name, path, False{})
52	    case other:
53	      Nil{}
54	
55	def check.go(ds: List<&2, Calls.Def>, +path: String, acc: List<&2, List<&2, F.Finding>>) -> List<&2, F.Finding>:
56	  match ds:
57	    case Nil{}:
58	      List.concat(&2, F.Finding, List.reverse(&2, List<&2, F.Finding>, acc))
59	    case Con{Calls.Def{+name, +sig, body}, rest}:
60	      check.go(rest, path,
61	        Lazy.stop(List<&2, F.Finding>, Bool.not(Bool.and(Calls.seq(sig), Bool.not(Calls.exempt(path, sig)))), [],
62	          _u => walk(body, name, path, False{})) <> acc)
63	
64	# the rule
65	def check(ss: Src.Src) -> List<&2, F.Finding>:
66	  Src.Src{path, text, toks, tree, bound, items} = ss
67	  check.go(Calls.defs(tree), path, [])