diff --git a/src/runtime-js.ts b/src/runtime-js.ts
index ec3ce21..db04c8a 100644
--- a/src/runtime-js.ts
+++ b/src/runtime-js.ts
@@ -80,6 +80,7 @@ export const _rt = {
         return String(value);
     },
     chars: (s: string) => s.split(''),
+    isWordChar: (s: string) => ({ _tag: /^\w$/.test(s) ? 'True' : 'False' }),
     // join: (delim: string) => (xs: string[]) => xs.join(delim),
     split: (delim: string) => (xs: string) => xs.split(delim),
     slice: (s: string | any[]) => (start: number) => (end: number) => s.slice(start, end),