setup hooks so that the behavior of the home / c:a keybinings delegates to a method to find where to go

then, use that to change how it works for the scheme mode (and also another variation for the REPL to
cope with the prompt)

I spent a while trying to make this work at the keymap% level (ie putting different keybindings for "home"
and "c:a" into different keymaps) but this just turned out to be far too confusing and fragile, so went
with this alternative (one keybinding, but that delegates to an overridable method)

closes PR 11446

original commit: d2cb96bcb39d93f6dac92906bed9885828663798

collects/framework/private/keymap.rkt

@@ -14,7 +14,8 @@
           [prefix finder: framework:finder^]
           [prefix handler: framework:handler^]
           [prefix frame: framework:frame^]
-          [prefix editor: framework:editor^])
+          [prefix editor: framework:editor^]
+          [prefix text: framework:text^])
   (export (rename framework:keymap^
                   [-get-file get-file]))
   (init-depend mred^)
@@ -1013,7 +1014,28 @@
                           (send text end-edit-sequence))))))))]
            
            [greek-letters "αβγδεζηθι κλμνξοπρςστυφχψω"]
-           [Greek-letters "ΑΒΓΔΕΖΗΘΙ ΚΛΜΝΞΟΠΡ ΣΤΥΦΧΨΩ"]) ;; don't have a capital ς, just comes out as \u03A2 (or junk) 
+           [Greek-letters "ΑΒΓΔΕΖΗΘΙ ΚΛΜΝΞΟΠΡ ΣΤΥΦΧΨΩ"]
+           ;; don't have a capital ς, just comes out as \u03A2 (or junk) 
+     
+           
+           [find-beginning-of-line
+            (λ (txt)
+              (cond
+                [(is-a? txt text:basic<%>)
+                 (send txt get-start-of-line (send txt get-start-position))]
+                [(is-a? txt text%)
+                 (send txt line-start-position (send txt position-line (send txt get-start-position)))]
+                [else #f]))]
+           [beginning-of-line
+             (λ (txt event)
+               (define pos (find-beginning-of-line txt))
+               (when pos
+                 (send txt set-position pos pos)))]
+           [select-to-beginning-of-line
+            (λ (txt event)
+              (define pos (find-beginning-of-line txt))
+              (when pos
+                (send txt set-position pos (send txt get-end-position))))])
       
       (λ (kmap)
         (let* ([map (λ (key func) 
@@ -1103,6 +1125,9 @@
           (add "mouse-popup-menu" mouse-popup-menu)
           
           (add "make-read-only" make-read-only)
+        
+          (add "beginning-of-line" beginning-of-line)
+          (add "selec-to-beginning-of-line" select-to-beginning-of-line)
           
           ; Map keys to functions
           
@@ -1442,13 +1467,13 @@
   (define global (make-object aug-keymap%))
   (define global-main (make-object aug-keymap%))
   (send global chain-to-keymap global-main #f)
-  (setup-global global-main)
   (generic-setup global-main)
+  (setup-global global-main)
   (define (get-global) global)
   
   (define file (make-object aug-keymap%))
-  (setup-file file)
   (generic-setup file)
+  (setup-file file)
   (define (-get-file) file)
   
   (define search (make-object aug-keymap%))

collects/framework/private/scheme.rkt

@@ -1171,6 +1171,26 @@
         [else 
          (insert-paren this)]))
     
+    (define/override (get-start-of-line pos)
+      (define para (position-paragraph pos))
+      (define para-start (paragraph-start-position para))
+      (define para-end (paragraph-end-position para))
+      (define first-non-whitespace 
+        (let loop ([i para-start])
+          (cond
+            [(= i para-end) #f]
+            [(char-whitespace? (get-character i))
+             (loop (+ i 1))]
+            [else i])))
+      (define new-pos 
+        (cond 
+          [(not first-non-whitespace) para-start]
+          [(= pos para-start) first-non-whitespace]
+          [(<= pos first-non-whitespace) para-start]
+          [else first-non-whitespace]))
+      new-pos)
+
+    
     (super-new)))
 
 (define -text-mode<%>
@@ -1409,7 +1429,7 @@
       
       (map-meta "c:space" "select-forward-sexp")
       (map-meta "c:t" "transpose-sexp")
-      
+
       ;(map-meta "c:m" "mark-matching-parenthesis")
       ; this keybinding doesn't interact with the paren colorer
       )

collects/framework/private/text.rkt

@@ -73,7 +73,8 @@
     move/copy-to-edit
     initial-autowrap-bitmap
     get-port-name
-    port-name-matches?))
+    port-name-matches?
+    get-start-of-line))
 
 (define basic-mixin
   (mixin (editor:basic<%> (class->interface text%)) (basic<%>)
@@ -533,9 +534,13 @@
                          ""
                          parent)))
     
+    (define/public (get-start-of-line pos)
+      (line-start-position (position-line pos)))
+    
     (super-new)
     (set-autowrap-bitmap (initial-autowrap-bitmap))))
 
+
 (define (hash-cons! h k v) (hash-set! h k (cons v (hash-ref h k '()))))
 
 (define first-line<%>
@@ -3974,7 +3979,6 @@ designates the character that triggers autocompletion
     (super-new)
     (setup-padding)))
 
-
 (define basic% (basic-mixin (editor:basic-mixin text%)))
 (define hide-caret/selection% (hide-caret/selection-mixin basic%))
 (define nbsp->space% (nbsp->space-mixin basic%))

collects/scribblings/framework/scheme.scrbl

@@ -234,6 +234,11 @@
     as the prefix for auto-completion.
 
   }
+  
+  @defmethod[#:mode override (get-start-of-line [pos exact-nonnegative-integer?]) exact-nonnegative-integer?]{
+     Returns the first non-whitespace character in the paragraph containing @racket[pos],
+     unless the position is already there, in which case it returns the first position of the paragraph.
+  }
 }
 @definterface[scheme:text-mode<%> ()]{
   The result of

collects/scribblings/framework/text.scrbl

@@ -141,6 +141,13 @@
      The number is updated in @xmethod[text% after-insert] and
      @xmethod[text% after-delete].
   }
+ 
+  @defmethod[(get-start-of-line [pos exact-nonnegative-integer?]) exact-nonnegative-integer?]{
+    This method is used by @racket[keymap:setup-global] to implement 
+    a keybinding for the @racket["home"] key and for @racket["c:a"].
+    
+    Its default implementation is @racket[(#,(method text% line-start-position) (#,(method text% position-line) pos))].
+  }
   
 }
 @defmixin[text:basic-mixin (editor:basic<%> text%) (text:basic<%>)]{