Add Nullable.isNullable (rescript-lang#227)

* add `Nullable.isNullable`

* update CHANGELOG.md

---------

Co-authored-by: Gabriel Nordeborn <[email protected]>
(cherry picked from commit d68e2be)

Author: 2 people

CHANGELOG.md

@@ -5,6 +5,7 @@
 - BREAKING: Fixes the type of `RegExp.Result.t` to be `array<option<string>>` instead of `array<string>`. https://github.com/rescript-association/rescript-core/pull/233.
 - BREAKING: Adds typed bindings to `Intl`, replacing the options type of `{..}` with records. https://github.com/rescript-association/rescript-core/pull/65
 - BREAKING: Align List api with other modules (`List.getBy` -> `List.find` etc.). https://github.com/rescript-association/rescript-core/pull/195
+- Add `Nullable.isNullable` function. https://github.com/rescript-association/rescript-core/pull/227
 - Remove some deps to Belt, Pervasives and Js. https://github.com/rescript-association/rescript-core/pull/226/commits
 - Fix: Expose Intl.Common. https://github.com/rescript-association/rescript-core/pull/197
 - Add `Array.join` and deprecate `Array.joinWith`. https://github.com/rescript-association/rescript-core/pull/205

src/Core__Nullable.res

@@ -4,6 +4,8 @@ external null: t<'a> = "#null"
 
 external undefined: t<'a> = "#undefined"
 
+external isNullable: t<'a> => bool = "#is_nullable"
+
 external make: 'a => t<'a> = "%identity"
 
 external toOption: t<'a> => option<'a> = "#nullable_to_opt"

src/Core__Nullable.resi

@@ -34,6 +34,27 @@ Console.log(undefined) // Logs `undefined` to the console.
 */
 external undefined: t<'a> = "#undefined"
 
+/**
+`isNullable(a)` returns `true` if `a` is null or undefined, `false` otherwise.
+
+## Examples
+
+```rescript
+let myStr = "Hello"
+let asNullable = myStr->Nullable.make
+
+// Can't do the below because we're now forced to check for nullability
+// myStr == asNullable
+
+// Check if asNullable is not null or undefined
+switch asNullable->Nullable.isNullable {
+| true => assert(false)
+| false => assert(true)
+}
+```
+*/
+external isNullable: t<'a> => bool = "#is_nullable"
+
 /**
 Creates a new nullable value from the provided value.
 This means the compiler will enforce null checks for the new value.