Fix return type for is_empty to be Bool, not String

Author: kyleect

README.md

@@ -67,7 +67,7 @@ See [/spec](./spec/) for more syntax examples.
 | --------------------------------------------------------- | ----------------------------------------------- |
 | `id(value: Value) -> Value`                               | Returns the string arugment passed to it        |
 | `noop() -> String`                                        | Returns the string "noop"                       |
-| `is_empty(value: String) -> String`                       | Checks if the given string is empty             |
+| `is_empty(value: String) -> Bool`                         | Checks if the given string is empty             |
 | `and(a: Bool, b: Bool) -> Bool`                           | Logical AND operation between two booleans      |
 | `or(a: Bool, b: Bool) -> Bool`                            | Logical OR operation between two booleans       |
 | `cond(cond: Bool, then: Value, else: Value) -> Bool`      | Conditional expression                          |

spec/valid/type_builtin_is_empty.expr

@@ -0,0 +1 @@
+(type is_empty)

spec/valid/type_builtin_is_empty.expr.disassembled

@@ -0,0 +1,5 @@
+VERSION 0800
+----
+0000 GET BUILTIN        14 == 'type'
+0003 GET BUILTIN         2 == 'is_empty'
+0006 CALL             (1 args)

spec/valid/type_builtin_is_empty.expr.interpreted

@@ -0,0 +1 @@
+Type<Fn(String) -> Bool>

src/builtins.rs

@@ -129,7 +129,7 @@ impl<'a> BuiltinFn<'a> {
             ty: Type::String,
             variadic: false,
         }],
-        return_type: Type::String,
+        return_type: Type::Bool,
         func: Self::is_empty,
     };