cmd/hakurei/json: friendly error messages

This change handles errors returned by encoding/json and prints significantly cleaner messages.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/hakurei/json.go

@@ -0,0 +1,60 @@
+package main
+
+import (
+	"encoding/json"
+	"errors"
+	"io"
+	"strconv"
+)
+
+// decodeJSON decodes json from r and stores it in v. A non-nil error results in a call to fatal.
+func decodeJSON(fatal func(v ...any), op string, r io.Reader, v any) {
+	err := json.NewDecoder(r).Decode(v)
+	if err == nil {
+		return
+	}
+
+	var (
+		syntaxError        *json.SyntaxError
+		unmarshalTypeError *json.UnmarshalTypeError
+
+		msg string
+	)
+
+	switch {
+	case errors.As(err, &syntaxError) && syntaxError != nil:
+		msg = syntaxError.Error() +
+			" at byte " + strconv.FormatInt(syntaxError.Offset, 10)
+
+	case errors.As(err, &unmarshalTypeError) && unmarshalTypeError != nil:
+		msg = "inappropriate " + unmarshalTypeError.Value +
+			" at byte " + strconv.FormatInt(unmarshalTypeError.Offset, 10)
+
+	default:
+		// InvalidUnmarshalError: incorrect usage, does not need to be handled
+		// io.ErrUnexpectedEOF: no additional error information available
+		msg = err.Error()
+	}
+
+	fatal("cannot " + op + ": " + msg)
+}
+
+// encodeJSON encodes v to output. A non-nil error results in a call to fatal.
+func encodeJSON(fatal func(v ...any), output io.Writer, short bool, v any) {
+	encoder := json.NewEncoder(output)
+	if !short {
+		encoder.SetIndent("", "  ")
+	}
+
+	if err := encoder.Encode(v); err != nil {
+		var marshalerError *json.MarshalerError
+		if errors.As(err, &marshalerError) && marshalerError != nil {
+			// this likely indicates an implementation error in hst
+			fatal("cannot encode json for " + marshalerError.Type.String() + ": " + marshalerError.Err.Error())
+			return
+		}
+
+		// UnsupportedTypeError, UnsupportedValueError: incorrect usage, does not need to be handled
+		fatal("cannot write json: " + err.Error())
+	}
+}