1	# rule twice: a list pattern in a def that calls itself, opening with the
2	# same literal twice (`case 10 <> 10 <> _:`, `case Con{'a', Con{'a', t}}:`).
3	# The checker hangs on it (pi-bend BEND-018, b6a83a52; bend 2.0.16 still
4	# does). Only the opening pair was seen to hang: `10 <> 11 <> 10`,
5	# `_ <> 10 <> 10` and `10 <> 11 <> 11` check at once, and so does the same
6	# pattern in a def that does not recurse. Match one element per step, with a
7	# state. A def recurses when its body calls it, `name(..)`: a parameter or a
8	# value named like the def is not a call. Only a case's first match column is
9	# read: a list pattern in a later column of `match a b:` is not.
10	import Base
11	import ../../src.bend as Src
12	import ../../finding.bend as F
13	import ../../syntax/lex.bend as Lex
14	import ../../syntax/tree.bend as Tree
15	import ../../syntax/bind.bend as Bind
16	import ../../lazy/lazy.bend as Lazy
17	import ../tokens.bend as T
18	import ../calls.bend as Calls
19	
20	# the literal a list pattern opens with: `x <> ..` or `Con{x, ..}`
21	def head(pat: Tree.Node) -> Maybe<&2, Lex.Tok>:
22	  match pat:
23	    case Tree.NCons{Tree.Leaf{Lex.Tok{+k, +t, l, c}}, Tree.NCons{Tree.Leaf{Lex.Tok{k2, +o, l2, c2}}, rest}}:
24	      Bool.pick(Maybe<&2, Lex.Tok>, Bool.and(T.is_lit(k), String.eq(o, "<>")), Some{Lex.Tok{k, t, l, c}}, None{})
25	    case Tree.NCons{Tree.Leaf{Lex.Tok{Lex.TUpper{}, +t, l, c}},
26	        Tree.NCons{Tree.Group{open, Tree.NCons{Tree.Leaf{Lex.Tok{+k, +x, l2, c2}}, kids}, close}, rest}}:
27	      Bool.pick(Maybe<&2, Lex.Tok>, Bool.and(String.eq(t, "Con"), T.is_lit(k)), Some{Lex.Tok{k, x, l2, c2}}, None{})
28	    case other:
29	      None{}
30	
31	# the rest of a list pattern after its first element
32	def tail(pat: Tree.Node) -> Tree.Node:
33	  match pat:
34	    case Tree.NCons{Tree.Leaf{Lex.Tok{Lex.TUpper{}, t, l, c}},
35	        Tree.NCons{Tree.Group{open, Tree.NCons{x, Tree.NCons{comma, kids}}, close}, rest}}:
36	      kids
37	    case Tree.NCons{x, Tree.NCons{op, rest}}:
38	      rest
39	    case other:
40	      Tree.NNil{}
41	
42	# a finding at the second literal when it repeats the first
43	def same(aa: Maybe<&2, Lex.Tok>, bb: Maybe<&2, Lex.Tok>, +path: String) -> List<&2, F.Finding>:
44	  match aa bb:
45	    case Some{Lex.Tok{k, +t, l, c}} Some{Lex.Tok{k2, +t2, l2, c2}}:
46	      Bool.pick(List<&2, F.Finding>, String.eq(t, t2),
47	        [F.Finding{path, l2, c2, T.width(t2), "twice",
48	          "This literal appears twice in one list pattern, which makes the checker hang; match one element per step."}],
49	        [])
50	    case a2 b2:
51	      []
52	
53	# a case statement's pattern, checked
54	def arm(+pat: Tree.Node, +path: String) -> List<&2, F.Finding>:
55	  same(head(pat), head(tail(pat)), path)
56	
57	# every case statement under a recursive def, at any depth
58	def walk(nn: Tree.Node, +path: String) -> List<&2, F.Finding>:
59	  match nn:
60	    case Tree.NCons{Tree.Stmt{Tree.SCase{}, kids, body}, rest}:
61	      List.concat(&2, F.Finding, [arm(T.pattern(kids), path), walk(body, path), walk(rest, path)])
62	    case Tree.NCons{Tree.Stmt{kind, kids, body}, rest}:
63	      List.append(&2, F.Finding, walk(body, path), walk(rest, path))
64	    case Tree.NCons{h, rest}:
65	      walk(rest, path)
66	    case other:
67	      Nil{}
68	
69	# a def's body, when it calls itself
70	def check.def(mm: Maybe<&2, String>, +body: Tree.Node, path: String) -> List<&2, F.Finding>:
71	  match mm:
72	    case None{}:
73	      Nil{}
74	    case Some{+name}:
75	      Lazy.stop(List<&2, F.Finding>, Bool.not(Calls.calls(body, name)), [], _u => walk(body, path))
76	
77	def check.go(root: Tree.Node, +path: String) -> List<&2, F.Finding>:
78	  match root:
79	    case Tree.NCons{Tree.Stmt{Tree.SDef{}, +kids, body}, rest}:
80	      List.append(&2, F.Finding, check.def(Bind.declared(kids), body, path), check.go(rest, path))
81	    case Tree.NCons{h, rest}:
82	      check.go(rest, path)
83	    case other:
84	      Nil{}
85	
86	# the rule
87	def check(ss: Src.Src) -> List<&2, F.Finding>:
88	  Src.Src{path, text, toks, tree, bound, items} = ss
89	  check.go(tree, path)