Resolve architect findings: gate [ on json.Valid, fix the root-relative README claim, test isTemplate

README.md

@@ -42,9 +42,10 @@ A path names a category, or a field inside one: each dot segment descends one
 level — folders, then the category (a JSON file), then fields. An argument that is
 a JSON object or array, or that carries a `{` token, is instead an **inline
 template**: a format string or a JSON value compiled and rendered on the spot. Its
-tokens reach the data by reference — `{/sv_SE.person.last}` from the root, `{.name}`
+tokens reach the data by reference from the root — `{/sv_SE.person.last}` (or
-and `{..name}` relative to it — so `--data-path` categories are available too. A
+`{.name}`, which means the same here), so shipped and `--data-path` categories are
-path never contains a brace, so the two cannot collide (see
+available — and `{..name}` is rejected, an inline template having no folder to step
+up from. A path never contains a brace, so the two cannot collide (see
 [Decisions](#decisions)).
 
 | Flag | |

cmd/fejkdata/main.go

@@ -9,6 +9,7 @@ package main
 
 import (
 	"bufio"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -243,13 +244,14 @@ func (in invocation) write(f *fejkdata.Generator, w io.Writer) error {
 }
 
 // isTemplate reports whether an argument is an inline template rather than a
-// path: a JSON object or array, or a format string carrying a { token. A path can
+// path: a format string carrying a { token (a path can never contain a brace),
-// never contain a brace, so the two never collide.
+// or a JSON object or array. A [ can begin a real category name, so a [
+// counts as a template only when the whole argument is valid JSON.
 func isTemplate(arg string) bool {
 	if strings.ContainsRune(arg, '{') {
 		return true
 	}
-	return strings.HasPrefix(arg, "[")
+	return strings.HasPrefix(arg, "[") && json.Valid([]byte(arg))
 }
 
 // renderArg renders one positional argument: an inline template, or a path.

cmd/fejkdata/main_test.go

@@ -301,6 +301,23 @@ func TestRunShippedDataByDefault(t *testing.T) {
 	}
 }
 
+func TestIsTemplate(t *testing.T) {
+	for arg, want := range map[string]bool{
+		"sv_SE.person":      false,
+		"person.last":       false,
+		"[abc]":             false, // a [ can open a real category name; not JSON
+		"[abc].field":       false,
+		"name: {x}":         true, // a { token: a path can never carry a brace
+		`{"format":"x"}`:    true,
+		`["a","b"]`:         true, // a JSON array carries no brace
+		`[1, 2]`:            true,
+	} {
+		if got := isTemplate(arg); got != want {
+			t.Errorf("isTemplate(%q) = %v, want %v", arg, got, want)
+		}
+	}
+}
+
 func TestRunInlineTemplate(t *testing.T) {
 	code, out, errb := runOut("--seed", "1", "name: {/sv_SE.person.last}")
 	if code != 0 || !strings.HasPrefix(out, "name: ") || strings.Contains(out, "{") {