feat: improve astToString, add tests for astToString (#873)

dev/configs/.changeset/pretty-dogs-film.md

@@ -0,0 +1,7 @@
+---
+"arktype": patch
+---
+
+Improve static parse error contextual summaries
+
+Thanks @Hsiwe! https://github.com/arktypeio/arktype/pull/873

dev/test/astUtils.test.ts

@@ -0,0 +1,20 @@
+import { describe, it } from "mocha"
+import type { astToString } from "../../src/parse/ast/utils.js"
+import { attest } from "../attest/main.js"
+
+describe("astToString", () => {
+    it("no parentheses if nested ast is an array", () => {
+        const t = {} as unknown as astToString<
+            [["number", "[]"], "|", "number"]
+        >
+        attest(t).typed as "'number[]|number'"
+    })
+    it("parentheses if nested ast is an infix expression", () => {
+        const t = {} as unknown as astToString<[["0", "|", "1"], "|", "string"]>
+        attest(t).typed as "'(0|1)|string'"
+    })
+    it('defaults to "..." if input is bad', () => {
+        const t = {} as unknown as astToString<["0", "///", "1"]>
+        attest(t).typed as "'...'"
+    })
+})

src/parse/ast/utils.ts

@@ -1,12 +1,26 @@
-import type { Literalable } from "../../utils/generics.js"
+import type { List, Literalable } from "../../utils/generics.js"
+import type { Scanner } from "../string/shift/scanner.js"
+import type { InfixExpression, PostfixExpression } from "./ast.js"
 
-export type astToString<ast> = `'${astToStringRecurse<ast, "">}'`
+export type astToString<ast> = `'${astToStringRecurse<ast>}'`
 
-type astToStringRecurse<ast, result extends string> = ast extends [
-    infer head,
-    ...infer tail
-]
-    ? astToStringRecurse<tail, `${result}${astToStringRecurse<head, "">}`>
+type astToStringRecurse<ast> = ast extends PostfixExpression<
+    infer operator,
+    infer operand
+>
+    ? operator extends "[]"
+        ? `${groupAst<operand>}[]`
+        : never
+    : ast extends InfixExpression<infer operator, infer l, infer r>
+    ? operator extends "&" | "|" | "%" | Scanner.Comparator
+        ? `${groupAst<l>}${operator}${groupAst<r>}`
+        : never
     : ast extends Literalable
-    ? `${result}${ast extends bigint ? `${ast}n` : ast}`
+    ? `${ast extends bigint ? `${ast}n` : ast}`
     : "..."
+
+type groupAst<ast> = ast extends List
+    ? ast[1] extends "[]"
+        ? astToStringRecurse<ast>
+        : `(${astToStringRecurse<ast>})`
+    : astToStringRecurse<ast>