feat(option): add expect

Author: glennsl

src/Core__Option.mjs

@@ -40,6 +40,17 @@ function getExn(x) {
       };
 }
 
+function expect(opt, message) {
+  if (opt !== undefined) {
+    return Caml_option.valFromOption(opt);
+  }
+  throw {
+        RE_EXN_ID: "Failure",
+        _1: message,
+        Error: new Error()
+      };
+}
+
 function mapWithDefault(opt, $$default, f) {
   var f$1 = Curry.__1(f);
   if (opt !== undefined) {
@@ -123,6 +134,7 @@ export {
   filter ,
   forEach ,
   getExn ,
+  expect ,
   mapWithDefault ,
   map ,
   flatMap ,

src/Core__Option.res

@@ -52,6 +52,12 @@ let getExn = x =>
   | None => raise(Not_found)
   }
 
+let expect = (opt, message) =>
+  switch opt {
+  | Some(value) => value
+  | None => raise(Failure(message))
+  }
+
 external getUnsafe: option<'a> => 'a = "%identity"
 
 let mapWithDefaultU = (opt, default, f) =>

src/Core__Option.resi

@@ -103,6 +103,20 @@ Option.getExn(None) /* Raises an Error */
 */
 let getExn: option<'a> => 'a
 
+/**
+`expect(opt, message)` returns `value` if `Some(value)`, raises a `Failure` expection with the given message if `None`.
+
+```rescript
+Option.expect(Some(3), "should not be None") // 3
+Option.expect(None, "should not be None") // Raises `Failure("should not be None")`
+```
+
+## Exceptions
+
+- Raises `Failure` if `opt` is `None`
+*/
+let expect: (option<'a>, string) => 'a
+
 /**
 `getUnsafe(value)` returns `value`.