preserve elseif syntax in parser

part of JuliaLang#21774

NEWS.md

@@ -32,6 +32,9 @@ Language changes
   * `{ }` expressions now use `braces` and `bracescat` as expression heads instead
     of `cell1d` and `cell2d`, and parse similarly to `vect` and `vcat` ([#8470]).
 
+  * Nested `if` expressions that arise from the keyword `elseif` now use `elseif`
+    as their expression head instead of `if` ([#21774]).
+
 Breaking changes
 ----------------
 

base/show.jl

@@ -556,7 +556,11 @@ function show_block(io::IO, head, args::Vector, body, indent::Int)
     print(io, head)
     if !isempty(args)
         print(io, ' ')
-        show_list(io, args, ", ", indent)
+        if head === :elseif
+            show_list(io, args, " ", indent)
+        else
+            show_list(io, args, ", ", indent)
+        end
     end
 
     ind = head === :module || head === :baremodule ? indent : indent + indent_width
@@ -879,10 +883,19 @@ function show_unquoted(io::IO, ex::Expr, indent::Int, prec::Int)
         print(io, "function ", args[1], " end")
 
     # block with argument
-    elseif head in (:for,:while,:function,:if) && nargs==2
+    elseif head in (:for,:while,:function,:if,:elseif) && nargs==2
         show_block(io, head, args[1], args[2], indent)
         print(io, "end")
 
+    elseif (head === :if || head === :elseif) && nargs == 3
+        show_block(io, head, args[1], args[2], indent)
+        if isa(args[3],Expr) && args[3].head == :elseif
+            show_unquoted(io, args[3], indent, prec)
+        else
+            show_block(io, "else", args[3], indent)
+            print(io, "end")
+        end
+
     elseif head === :module && nargs==3 && isa(args[1],Bool)
         show_block(io, args[1] ? :module : :baremodule, args[2], args[3], indent)
         print(io, "end")
@@ -944,11 +957,6 @@ function show_unquoted(io::IO, ex::Expr, indent::Int, prec::Int)
     elseif head === :line && 1 <= nargs <= 2
         show_linenumber(io, args...)
 
-    elseif head === :if && nargs == 3     # if/else
-        show_block(io, "if",   args[1], args[2], indent)
-        show_block(io, "else", args[3], indent)
-        print(io, "end")
-
     elseif head === :try && 3 <= nargs <= 4
         show_block(io, "try", args[1], indent)
         if is_expr(args[3], :block)

doc/src/devdocs/ast.md

@@ -466,18 +466,17 @@ if a
     b
 elseif c
     d
-else e
-    f
+else
+    e
 end
 ```
 
 parses as:
 
 ```
 (if a (block (line 2) b)
-    (block (line 3) (if c (block (line 4) d)
-                             (block (line 5) e
-                                    (line 6) f))))
+    (elseif (block (line 3) c) (block (line 4) d)
+            (block (line 5 e))))
 ```
 
 A `while` loop parses as `(while condition body)`.

src/julia-parser.scm

@@ -1291,31 +1291,33 @@
           `(for ,(if (length= ranges 1) (car ranges) (cons 'block ranges))
                 ,body)))
 
-       ((if)
+       ((if elseif)
         (if (newline? (peek-token s))
             (error (string "missing condition in \"if\" at " current-filename
                            ":" (- (input-port-line (ts:port s)) 1))))
-        (let* ((test (parse-cond s))
+        (let* ((lno (line-number-node s))  ;; line number for elseif condition
+               (test (parse-cond s))
+               (test (if (eq? word 'elseif)
+                         `(block ,lno ,test)
+                         test))
                (then (if (memq (require-token s) '(else elseif))
                          '(block)
                          (parse-block s)))
                (nxt  (require-token s)))
           (take-token s)
           (case nxt
-            ((end)     (list 'if test then))
+            ((end)     (list word test then))
             ((elseif)
              (if (newline? (peek-token s))
                  (error (string "missing condition in \"elseif\" at " current-filename
                                 ":" (- (input-port-line (ts:port s)) 1))))
-             `(if ,test ,then
-                  ;; line number for elseif condition
-                  (block ,(line-number-node s)
-                         ,(parse-resword s 'if))))
+             `(,word ,test ,then
+                     ,(parse-resword s 'elseif)))
             ((else)
              (if (eq? (peek-token s) 'if)
                  (error "use \"elseif\" instead of \"else if\""))
-             (begin0 (list 'if test then (parse-block s))
-                     (expect-end s word)))
+             (begin0 (list word test then (parse-block s))
+                     (expect-end s 'if)))
             (else      (error (string "unexpected \"" nxt "\""))))))
        ((let)
         (let ((binds (if (memv (peek-token s) '(#\newline #\;))

src/julia-syntax.scm

@@ -1669,15 +1669,6 @@
                             (if ,g ,g
                                 ,(loop (cdr tail)))))))))))
 
-(define (expand-forms e)
-  (if (or (atom? e) (memq (car e) '(quote inert top core globalref outerref line module toplevel ssavalue null meta)))
-      e
-      (let ((ex (get expand-table (car e) #f)))
-        (if ex
-            (ex e)
-            (cons (car e)
-                  (map expand-forms (cdr e)))))))
-
 (define (expand-for while lhs X body)
   ;; (for (= lhs X) body)
   (let ((coll  (make-ssavalue))
@@ -1884,6 +1875,15 @@
               (extract (cdr params) (cons p newparams) whereparams)))))
   (extract (cddr e) '() '()))
 
+(define (expand-forms e)
+  (if (or (atom? e) (memq (car e) '(quote inert top core globalref outerref line module toplevel ssavalue null meta)))
+      e
+      (let ((ex (get expand-table (car e) #f)))
+        (if ex
+            (ex e)
+            (cons (car e)
+                  (map expand-forms (cdr e)))))))
+
 ;; table mapping expression head to a function expanding that form
 (define expand-table
   (table
@@ -3502,7 +3502,7 @@ f(x) = yt(x)
              (if value
                  (compile (cadr e) break-labels value tail)
                  #f))
-            ((if)
+            ((if elseif)
              (let ((test `(gotoifnot ,(compile-cond (cadr e) break-labels) _))
                    (end-jump `(goto _))
                    (val (if (and value (not tail)) (new-mutable-var) #f)))

test/show.jl

@@ -236,6 +236,45 @@ end"""
     return
 end"""
 
+@test_repr """if a
+# line meta
+b
+end
+"""
+
+@test_repr """if a
+# line meta
+b
+elseif c
+# line meta
+d
+end
+"""
+
+@test_repr """if a
+# line meta
+b
+elseif c
+# line meta
+d
+else
+# line meta
+e
+end
+"""
+
+@test_repr """if a
+# line meta
+b
+elseif c
+# line meta
+d
+elseif e
+# line meta
+f
+end
+"""
+
 # issue #7188
 @test sprint(show, :foo) == ":foo"
 @test sprint(show, Symbol("foo bar")) == "Symbol(\"foo bar\")"