String.unsafeGet (#223)

src/Core__String.res

@@ -13,6 +13,7 @@ let compare = (a: string, b: string) =>
 
 @get external length: string => int = "length"
 @get_index external get: (string, int) => option<string> = ""
+@get_index external getUnsafe: (string, int) => string = ""
 @send external charAt: (string, int) => string = "charAt"
 
 @send external charCodeAt: (string, int) => float = "charCodeAt"

src/Core__String.resi

@@ -150,6 +150,22 @@ String.get(`JS`, 4) == None
 @get_index
 external get: (string, int) => option<string> = ""
 
+/**
+`getUnsafe(str, index)` returns an `string` at the given `index` number.
+
+This is _unsafe_, meaning it will return `undefined` value if `index` does not exist in `string`.
+
+Use `String.getUnsafe` only when you are sure the `index` exists.
+## Examples
+
+```rescript
+String.getUnsafe("ReScript", 0) == "R"
+String.getUnsafe("Hello", 4) == "o"
+```
+*/
+@get_index
+external getUnsafe: (string, int) => string = ""
+
 /**
 `charAt(str, index)` gets the character at `index` within string `str`. If
 `index` is negative or greater than the length of `str`, it returns the empty