improve a message

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/wasm/wasm.rs b/wasm/wasm.rs
index af7a2c7e08aa38fe618d2934b93b9ffb72e3a159..8f4315119d7d952fd744d48013661e3e6b8b1a8b 100644 (file)
--- a/wasm/wasm.rs
+++ b/wasm/wasm.rs
@@ -31,7 +31,8 @@ impl<E:Display, V> WasmResult<V> for Result<V, E> {
 #[wasm_bindgen]
 pub fn check(packed: &JsValue) {
   let s = packed.as_string().ok_or_else(
-    ||"packed Z coordinate wrong JS type (not a string)"
+    ||format!("packed Z coordinate wrong JS type (not a string): {:?}",
+              packed)
   ).e()?;
   ZCoord::check_str(&s).ok_or(zcoord::ParseError).e()?;
 }