Export ErrNoColumns, name the cure for a shell-split template and an unquoted record, and name the flag a missing data path came from

builtins.go

@@ -225,7 +225,7 @@ func floatArgs(_ map[string]node, a []string) error {
 }
 func seqArg(_ map[string]node, a []string) error {
 	if len(a) > 1 {
-		return fmt.Errorf("seq takes at most one name, got %d args", len(a))
+		return fmt.Errorf("seq takes at most one name, got %d", len(a))
 	}
 	if len(a) == 1 && a[0] == "" {
 		return fmt.Errorf("seq name must not be empty")

cmd/fejkdata/main.go

@@ -282,7 +282,7 @@ func (in invocation) check() (argKind, error) {
 		return argPath, nil
 	}
 	if len(in.paths) != 1 {
-		return argPath, fmt.Errorf("expected one path or template, got %d", len(in.paths))
+		return argPath, fmt.Errorf("expected one path or template, got %d%s", len(in.paths), templateSplit(in.paths))
 	}
 	return classify(in.paths[0])
 }
@@ -478,11 +478,25 @@ func run(args []string, stdout, stderr io.Writer) int {
 			return misuse(stderr, te.error)
 		}
 		fmt.Fprintln(stderr, err)
+		if errors.Is(err, fejkdata.ErrNoColumns) {
+			fmt.Fprintln(stderr, "wrap that JSON in single quotes, which keep a shell from expanding its braces")
+		}
 		return 1
 	}
 	return 0
 }
 
+// templateSplit names the cure for a template a shell split on its spaces, which is
+// what several arguments carrying a token mean.
+func templateSplit(paths []string) string {
+	for _, p := range paths {
+		if strings.Contains(p, "{") {
+			return `; a template's spaces split the argument, so wrap the whole argument in "…"`
+		}
+	}
+	return ""
+}
+
 func misuse(stderr io.Writer, err error) int {
 	// A library error already names the program, so the prefix is not doubled.
 	fmt.Fprintf(stderr, "fejkdata: %s\ntry 'fejkdata --help'\n", strings.TrimPrefix(err.Error(), "fejkdata: "))

data.go

@@ -43,7 +43,7 @@ func loadData(sources []dataSource) (map[string]node, error) {
 			}
 			info, err := os.Stat(src.path)
 			if err != nil {
-				return nil, err
+				return nil, fmt.Errorf("data path %s: %w", src.path, err)
 			}
 			if !info.IsDir() {
 				return nil, fmt.Errorf("%s is not a directory", src.path)

record.go

@@ -140,7 +140,7 @@ func (f *Generator) FakeRecord(path string) (*Record, error) {
 		return nil, fmt.Errorf("fejkdata: %s descends into %q, a field; only a category-level template is a record", path, tail[0])
 	}
 	shape := f.recordShapeOf(n)
-	if errors.Is(shape.err, errNoColumns) {
+	if errors.Is(shape.err, ErrNoColumns) {
 		ns := names(segments)
 		return nil, fmt.Errorf(`fejkdata: %s %w; render it as a column of one: {"format":"","%s":"{/%s}"}`, path, shape.err, ns[len(ns)-1], path)
 	}
@@ -231,9 +231,9 @@ func (f *Generator) FakeRecordTemplate(input string) (*Record, error) {
 	return t.Fake(), nil
 }
 
-// errNoColumns is the one record fence a path can answer, so its entry point names
+// ErrNoColumns is the one record fence a path can answer, so its entry point names
 // the record to write instead.
-var errNoColumns = errors.New("has no fields, so no columns")
+var ErrNoColumns = errors.New("has no fields, so no columns")
 
 // recordOf is the fence both record entry points pass. The columns come back with
 // the template, fixed for every draw the caller goes on to make.
@@ -247,7 +247,7 @@ func recordOf(n node) (*template, []Column, error) {
 	}
 	names := recordColumns(t)
 	if len(names) == 0 {
-		return nil, nil, errNoColumns
+		return nil, nil, ErrNoColumns
 	}
 	if !t.record {
 		return nil, nil, fmt.Errorf("carries repeat %d, which composes its format into one string; a record projects columns instead — drop the repeat and render the record again for more rows", t.repeat)

struct.go

@@ -340,7 +340,7 @@ func (p *valueProof) checkField(label string, ft reflect.Type, column node) erro
 			return fmt.Errorf("%s (%s): %s", label, ft, reason)
 		}
 		if !kind.holds(v) {
-			return fmt.Errorf("%s (%s): %q is not proven within %s; make it %s", label, ft, it.format, elem.Kind(), kind.wider)
+			return fmt.Errorf("%s (%s): %q is not proven within %s; narrow it to that range, or make the field %s", label, ft, it.format, elem.Kind(), kind.wider)
 		}
 	}
 	return nil