lisp-koans/koans/special-forms.lsp

;;   Copyright 2013 Google Inc.
;;
;;   Licensed under the Apache License, Version 2.0 (the "License");
;;   you may not use this file except in compliance with the License.
;;   You may obtain a copy of the License at
;;
;;       http://www.apache.org/licenses/LICENSE-2.0
;;
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;;   distributed under the License is distributed on an "AS IS" BASIS,
;;   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
;;   See the License for the specific language governing permissions and
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; Special forms are evaluatable lisp forms (lists) which are
; neither functions nor macros.  Here is an introduction to a
; few of them.

; based on http://psg.com/~dlamkins/sl/chapter03-03.html

(defvar my-name)
(defvar my-clones-name)
(defvar a)
(defvar b)
(defvar c 0)

(define-test test-setf
    "setf is used to assign values to symbols.  These symbols may refer to
     variables with lexical or dynamic scope."
  (setf my-name "David")
  (assert-equal my-name "David")
  " In SBCL, if the symbol isn't defined as a variable, via a top-level defvar
  or let statement, the setf call may result in a warning."
  (setf my-clones-name my-name)
  (assert-equal "David" my-clones-name)
  (setf a 5)
  (setf b 10)
  (setf c 50)
  (assert-equal 50 c))


(define-test test-let
    "The let form establishes a lexical extent, within which explicit symbols
     may be bound to values.  The binding only extends over the extent of the
     lexical form.  After which, the previous value, if it exists, is visible again."
  (setf a 10)
  (setf b 20)
  (assert-equal a 10)
  (assert-equal b 20)
  (let ((a 1111)
        (b 2222))
    (assert-equal a 1111)
    (assert-equal b 2222))
  (assert-equal a 10)
  (assert-equal b 20))


(define-test test-let-default-value
    "let vars have a default value"
    (let ((x))
      (assert-equal nil x)))

(define-test test-let-bindings-are-parallel
    "When defining the bindings in the let form, later bindings may not depend
     on earlier ones"
  (setf a 100)
  (let ((a 5)
        (b (* 10 a)))
    (assert-equal b 1000)))

(define-test test-let*-bindings-are-series
    "let* is like let, but successive bindings may use values of previous ones"
  (setf a 100)
  (let* ((a 5)
         (b (* 10 a)))
    (assert-equal b 50))
  (assert-equal a 100))


(define-test write-your-own-let-statement
    "fix the let statement to get the tests to pass"
  (setf a 100)
  (setf b 23)
  (setf c 456)
  (let ((a a)
        (b (* a 2))
        (c "Jellyfish"))
    (assert-equal a 100)
    (assert-equal b 200)
    (assert-equal c "Jellyfish"))
  (let* ((a 121)
         (b 200)
         (c (+ a (/ b a))))
    (assert-equal a 121)
    (assert-equal b 200)
    (assert-equal c (+ a (/ b a)))))


(define-test test-cond
    "the cond form is like the c switch statement"
  (setf a 4)
  (setf c
        (cond ((> a 0) :positive)
              ((< a 0) :negative)
              (t :zero)))
  (assert-equal :positive c))


(defun cartoon-dads (input)
  " you should be able to complete this cond statement"
  (cond ((equal input :this-one-doesnt-happen) :fancy-cat)
        ((equal input :bart) :homer)
        ((equal input :stewie) :peter)
        ((equal input :stan) :randy)
        (t :unknown)))

(define-test test-your-own-cond-statement
    "fix this by completing the 'cartoon-dads' function above"
  (assert-equal (cartoon-dads :bart) :homer)
  (assert-equal (cartoon-dads :stewie) :peter)
  (assert-equal (cartoon-dads :stan) :randy)
  (assert-equal (cartoon-dads :space-ghost) :unknown))