Merge branch 'cond' into 'main'

Added cond function to Userlib

See merge request whom/relish!8

Writing.org

@@ -126,6 +126,29 @@ Example:
      (eq? (some-big-calculation) expected-result))
 #+END_SRC
 
+*** Cond
+*Cond* is a function defined in the *Userlib* that acts as syntax sugar for nested *if form*s.
+Given a list of pairs consisting of a condition and a form to execute *cond* will iterate trough the list evaluating the conditions in order, Upon encountering a condition that evaluates to ~true~ the corresponding form will be evaluated and its result returned, thus halting the loop so no further forms are evaluated.
+If no conditions evaluate to true then *cond* won't execute anything
+The argument to *cond* must be given using *quote*, otherwise it will be evaluated before being passed to *cond*, thus making it not work.
+
+Example:
+#+BEGIN_SRC lisp
+  (let ((list (1 2 3 4)))
+    (cond (q
+	   (((gt? (car list) 2) (echo "The first number of this list is greater than 2")) ;; The first condition returns false so this expression won't be evaluated
+	    ((gt? (len list) 3) (echo "This list's length is greater than 3" )) ;; Since the second condition returns true this form will be evaluated and its result will be returned
+	    (true (echo "This list is rather unremarkable")))))) ;; This form will be evaluated if none of the previous conditions return true
+
+  ;; This is what the equivalent if form would look like
+  (let ((list (1 2 3 4)))
+    (if (gt? (car list) 2)
+	(echo "The first number of this list is greater than 2")
+	(if (gt? (len list) 3)
+	    (echo "This list's length is greater than 3")
+	    (echo "This list is rather unremarkable"))))
+#+END_SRC
+
 *** Not quite control flow
 Several other functions use lazy evaluation of their arguments. The below list is non-exhaustive:
 - toggle
@@ -274,7 +297,7 @@ The following table is up to date as of Relish 0.3.0. For latest information try
 |----------------+---------------+----------------+---------+--------+-----------+-----------+-----------+--------|
 |                | set?          | fg             | dq      | div    | concat    | or        | get-paths |        |
 |----------------+---------------+----------------+---------+--------+-----------+-----------+-----------+--------|
-|                |               |                | pop     | gte?   | string    |           |           |        |
+|                |               |                | pop     | gte?   | string    |           | cond      |        |
 |----------------+---------------+----------------+---------+--------+-----------+-----------+-----------+--------|
 |                |               |                |         | int    |           |           |           |        |
 |----------------+---------------+----------------+---------+--------+-----------+-----------+-----------+--------|

snippets/userlib-tests.rls

@@ -55,6 +55,36 @@
              (adder (lambda (x y) (add x y))))
          (eq? (reduce adder list) (add 1 2 3)))))
 
+   ('cond evaluates the first branch that returns true'
+     (quote
+       (let ((switch-one false)
+             (switch-two false)
+	     (switch-three false))
+         (cond (q
+             ((false (toggle switch-one))
+              (true (toggle switch-two))
+	      (true (toggle switch-three)))))
+	 (and (not switch-one) switch-two (not switch-three)))))
+
+   ('cond doesnt do anything if all the branches are false'
+     (quote
+      (let ((switch-one false)
+            (switch-two false)
+	    (switch-three false))
+       (cond (q
+           ((false (toggle switch-one))
+	    (false (toggle switch-two))
+	    (false (toggle switch-three)))))
+       (and (not switch-one) (not switch-two) (not switch-three)))))
+
+   ('cond returns the result of the branch that is evaluated'
+	  (quote
+	   (let ((variable false))
+	     (set (q variable)
+		  (cond (q
+			 ((true true)))))
+	     variable)))
+
    ('contains? finds elem in list'
      (quote
        (contains? (1 2 3) 1)))

snippets/userlib.rls

@@ -84,6 +84,20 @@ this will continue iuntil the list is exhausted.'
           (set (q list-iter) (pop remaining)))))
   result))
 
+(def cond
+'Takes one argument: a list of pairs consisting of a form returning a bool and a form to execute
+The function iterates over the list checking if the first form of a pair evaluates to true, in which
+case it will execute the second form of the pair, returning its result, and stop the loop.'
+(list)
+  (let ((iter list))
+    (while (gt? (len iter) 0)
+      (let ((current (car iter))
+	    (remaining (pop iter)))
+	(if (eval (car current))
+	    (car (pop ((set (q iter) ())
+		       (eval (cdr current)))))
+	    (set (q iter) (cdr remaining)))))))
+
 (def contains?
 'Takes two arguments: a list and an element.
 Returns true if the list contains the element.'