def head

bolt@1.27.0.0 · src/rules/correctness/twice.bend:20

import Base
import bolt@1.27.0.0/src/syntax/lex.bend as Lex
import bolt@1.27.0.0/src/syntax/tree.bend as Tree
import bolt@1.27.0.0/src/rules/tokens.bend as T

20# the literal a list pattern opens with: `x <> ..` or `Con{x, ..}`
21def 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{}