Add Nullable.isNullable (#227)

* add `Nullable.isNullable`

* update CHANGELOG.md

---------

Co-authored-by: Gabriel Nordeborn <[email protected]>

Author: aspeddro

Diff for: CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Next version
 
+- 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
 
 ## 1.3.0

Diff for: src/Core__Nullable.res

@@ -7,6 +7,8 @@ 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"

Diff for: src/Core__Nullable.resi

@@ -38,6 +38,27 @@ 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.