feat(nullable): add flatMap

Author: glennsl

src/Core__Nullable.mjs

@@ -44,11 +44,19 @@ function mapWithDefault(value, $$default, f) {
   }
 }
 
+function flatMap(value, f) {
+  if (!(value == null)) {
+    return Curry._1(f, value);
+  }
+  
+}
+
 export {
   fromOption ,
   getWithDefault ,
   getExn ,
   map ,
   mapWithDefault ,
+  flatMap ,
 }
 /* No side effect */

src/Core__Nullable.res

@@ -39,3 +39,9 @@ let mapWithDefault = (value, default, f) =>
   | Some(x) => f(x)
   | None => default
   }
+
+let flatMap = (value, f) =>
+  switch value->toOption {
+  | Some(x) => f(x)
+  | None => undefined
+  }