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hash-table-p

hash-table-p Function

Syntax:

hash-table-p object → generalized-boolean

Arguments and Values:

object—an object.

generalized-boolean—a generalized boolean.

Description:

Returns true if object is of type hash-table; otherwise, returns false.

Hash

Examples:

(setq table (make-hash-table)) → #<HASH-TABLE EQL 0/120 32511220> 
(hash-table-p table) → true 
(hash-table-p 37) → false 
(hash-table-p ’((a . 1) (b . 2))) → false

Notes:

(hash-table-p object) ≡ (typep object ’hash-table)

Expanded Reference: hash-table-p​

Basic Type Predicate​

hash-table-p returns true if its argument is a hash table, false otherwise.

(hash-table-p (make-hash-table))
=> T

(hash-table-p 42)
=> NIL

(hash-table-p "hello")
=> NIL

Not Confused by Similar Structures​

Association lists and property lists are not hash tables.

(hash-table-p '((a . 1) (b . 2) (c . 3)))
=> NIL

(hash-table-p '(a 1 b 2 c 3))
=> NIL

(hash-table-p nil)
=> NIL

Works with All Hash Table Variants​

(hash-table-p (make-hash-table :test #'eq))
=> T

(hash-table-p (make-hash-table :test #'equal))
=> T

(hash-table-p (make-hash-table :test #'equalp))
=> T

Equivalent to typep​

;; hash-table-p is equivalent to (typep object 'hash-table)
(let ((ht (make-hash-table)))
  (eq (hash-table-p ht)
      (typep ht 'hash-table)))
=> T

Useful in Defensive Programming​

(defun safe-lookup (key table)
  "Look up KEY in TABLE, signaling an error if TABLE is not a hash table."
  (unless (hash-table-p table)
    (error "Expected a hash table, got ~S" table))
  (gethash key table))

(safe-lookup 'x (make-hash-table))
=> NIL
=> NIL