add 'else' support to 2dcond

closes PR 13714

collects/tests/unstable/2d/cond-test.rkt

@@ -81,3 +81,24 @@
                 "1\n2\n3\n4\n"))
 
 
+(define (try-else a b c)
+  #2dcond
+  ╔════╦════╦════╗
+  ║    ║ a  ║else║
+  ╠════╬════╬════╣
+  ║ b  ║ 1  ║  2 ║
+  ╠════╬════╬════╣
+  ║ c  ║ 3  ║  4 ║
+  ╠════╬════╬════╣
+  ║else║ 5  ║  6 ║
+  ╚════╩════╩════╝)
+
+(require rackunit)
+(check-equal? (try-else #t #t #f) 1)
+(check-equal? (try-else #f #t #f) 2)
+(check-equal? (try-else #t #f #f) 5)
+(check-equal? (try-else #f #f #f) 6)
+
+
+
+

collects/unstable/2d/cond.rkt

@@ -1,4 +1,5 @@
-#lang racket
+#lang racket/base
+(require (for-syntax racket/base))
 
 (provide 2dcond)
 (define-syntax (2dcond stx)
@@ -7,6 +8,9 @@
         [(cell ...) rhs ...] ...)
      (let ()
        
+       (define last-col (- (length (syntax->list #'widths)) 1))
+       (define last-row (- (length (syntax->list #'heights)) 1))
+       
        ;; coord-to-content : hash[(list num num) -o> (listof syntax)]
        (define coord-to-content (make-hash))
        (define let-bindings '()) 
@@ -35,6 +39,16 @@
          
          (cond
            [(member (list 0 0) cells) (void)]
+           [(and (or (member (list 0 last-row) cells)
+                     (member (list last-col 0) cells))
+                 (syntax-case rhses (else)
+                   [(else) #t]
+                   [_ #f]))
+            ;; found an 'else' (in a reasonable place)
+            ;; => treat it like a #t in that cell
+            (hash-set! coord-to-content 
+                       (car cells)
+                       (list #'#t))]
            [(and 
              ;; only one cell:
              (null? (cdr cells))

collects/unstable/scribblings/2d.scrbl

@@ -90,9 +90,15 @@ the same.
 @defform/subs[(2dcond cond-content)
               ([cond-content (code:line question-row
                                         body-row
-                                        ⋮)]
-               [question-row (code:line empty-cell question-cell ⋯)]
+                                        ⋮)
+                             (code:line question-row
+                                        body-row
+                                        ⋮
+                                        else-row)]
+               [question-row (code:line empty-cell question-cell ⋯)
+                             (code:line empty-cell question-cell ⋯ else-cell)]
                [body-row (code:line question-cell exprs-cell ⋯)]
+               [else-row (code:line question-cell exprs-cell ⋯ else-cell)]
                [question-cell (code:line ╔═════════════╗
                                          ║question-expr║
                                          ╚═════════════╝)]
@@ -103,7 +109,10 @@ the same.
                
                [exprs-cell (code:line ╔═════════════╗
                                       ║expr expr ...║
-                                      ╚═════════════╝)])]{
+                                      ╚═════════════╝)]
+               [else-cell (code:line ╔══════╗
+                                     ║ else ║
+                                     ╚══════╝)])]{
   Evaluates the first row of question expressions until 
   one of them returns a true value (signaling an error if none do),
   then evaluates the first column of question expressions until