KSL Wiki

Has

Has[symbol]

Has[object, key]

A polymorphic function that checks for existence in the current environment or within an object.

• Single argument: Returns #t if symbol is defined in the current scope, otherwise #f.
• Two arguments: Returns #t if key exists in object, otherwise #f.

(* Symbol existence *)
Has[Has];  (* => #t *)
Has[a];    (* => #f *)

Let[a, 0];
Has[a];    (* => #t *)

(* Object key existence *)
Let[obj, Object[A, { { #a, 1 } }]];
Has[obj, #a];  (* => #t *)
Has[obj, #b];  (* => #f *)

Head

Head[list]

Retrieves the first element of list.

• Returns the first element of the list.
• An error is raised if list is empty.

Head[{ 1, 2, 3 }];        (* => 1 *)
Head[{ { #a, #b }, #c }]; (* => {#a, #b} *)

Head[{ }];
(*? Error[ksl::builtin::Head]: Empty list. *)