add try/catch/else (JuliaLang#42211)

This PR allows the use of `else` inside try-blocks, which is only taken
if no exception was caught inside `try`. It is still combinable with
`finally` as well.

If an error is thrown inside the `else` block, the current semantics are
that the error does not get caught and is thrown like normal, but the
`finally` block is still run afterwards. This seemed like the most
sensible option to me.

I am not very confident about the implementation of linearization for
`trycatchelse` here, so I would appreciate it if @JeffBezanson could
give this a thorough review, so that I don't miss any edge cases.

I thought we had an issue for this already, but I couldn't find
anything. `else` might also not be the best keyword here, so maybe we
can come up with something clearer. But it of course has the advantage
that it is already a Julia keyword, so we don't need to add a new one.

Co-authored-by: Jeff Bezanson <jeff.bezanson@gmail.com>

NEWS.md

@@ -13,6 +13,8 @@ New language features
 * The default behavior of observing `@inbounds` declarations is now an option via `auto` in `--check-bounds=yes|no|auto` ([#41551])
 * New function `eachsplit(str)` for iteratively performing `split(str)`.
 * `∀`, `∃`, and `∄` are now allowed as identifier characters ([#42314]).
+* `try`-blocks can now optionally have an `else`-block which is executed right after the main body only if
+  no errors were thrown. ([#42211])
 
 Language changes
 ----------------

base/show.jl

@@ -2132,12 +2132,15 @@ function show_unquoted(io::IO, ex::Expr, indent::Int, prec::Int, quote_level::In
     elseif head === :line && 1 <= nargs <= 2
         show_linenumber(io, args...)
 
-    elseif head === :try && 3 <= nargs <= 4
+    elseif head === :try && 3 <= nargs <= 5
         iob = IOContext(io, beginsym=>false)
         show_block(iob, "try", args[1], indent, quote_level)
         if is_expr(args[3], :block)
             show_block(iob, "catch", args[2] === false ? Any[] : args[2], args[3]::Expr, indent, quote_level)
         end
+        if nargs >= 5 && is_expr(args[5], :block)
+            show_block(iob, "else", Any[], args[5]::Expr, indent, quote_level)
+        end
         if nargs >= 4 && is_expr(args[4], :block)
             show_block(iob, "finally", Any[], args[4]::Expr, indent, quote_level)
         end

src/ast.scm

@@ -209,6 +209,13 @@
                                 "\n"
                                 (indented-block (cdr (cadddr e)) ilvl))
                         "")
+                    (if (length> e 5)
+                        (let ((els (cadddddr e)))
+                          (if (and (pair? els) (eq? (car els) 'block))
+                              (string (string.rep "    " ilvl) "else\n"
+                                      (indented-block (cdr els) ilvl))
+                              ""))
+                        "")
                     (if (length> e 4)
                         (let ((fin (caddddr e)))
                           (if (and (pair? fin) (eq? (car fin) 'block))

src/julia-parser.scm

@@ -1509,29 +1509,33 @@
           (let loop ((nxt    (peek-token s))
                      (catchb #f)
                      (catchv #f)
-                     (finalb #f))
+                     (finalb #f)
+                     (elseb #f))
             (take-token s)
             (cond
              ((eq? nxt 'end)
               (list* 'try try-block (or catchv '(false))
                      (or catchb (if finalb '(false) (error "try without catch or finally")))
-                     (if finalb (list finalb) '())))
+                     (cond (elseb  (list (or finalb '(false)) elseb))
+                           (finalb (list finalb))
+                           (else   '()))))
              ((and (eq? nxt 'catch)
                    (not catchb))
               (let ((nl (memv (peek-token s) '(#\newline #\;))))
                 (if (eqv? (peek-token s) #\;)
                     (take-token s))
-                (if (memq (require-token s) '(end finally))
+                (if (memq (require-token s) '(end finally else))
                     (loop (require-token s)
                           '(block)
                           #f
-                          finalb)
+                          finalb
+                          elseb)
                     (let* ((loc (line-number-node s))
                            (var (if nl #f (parse-eq* s)))
                            (var? (and (not nl) (or (symbol? var)
                                                    (and (length= var 2) (eq? (car var) '$))
                                                    (error (string "invalid syntax \"catch " (deparse var) "\"")))))
-                           (catch-block (if (eq? (require-token s) 'finally)
+                           (catch-block (if (memq (require-token s) '(finally else))
                                             `(block ,(line-number-node s))
                                             (parse-block s))))
                       (loop (require-token s)
@@ -1543,16 +1547,30 @@
                                               '()
                                               (cdr catch-block))))
                             (if var? var '(false))
-                            finalb)))))
+                            finalb
+                            elseb)))))
              ((and (eq? nxt 'finally)
                    (not finalb))
-              (let ((fb (if (eq? (require-token s) 'catch)
+              (let ((fb (if (memq (require-token s) '(catch else))
                             '(block)
                             (parse-block s))))
                 (loop (require-token s)
                       catchb
                       catchv
-                      fb)))
+                      fb
+                      elseb)))
+             ((and (eq? nxt 'else)
+                   (not elseb))
+              (if (or (not catchb) finalb)
+                  (error "else inside try block needs to be immediately after catch"))
+              (let ((eb (if (eq? (require-token s) 'finally)
+                            '(block)
+                            (parse-block s))))
+                (loop (require-token s)
+                      catchb
+                      catchv
+                      finalb
+                      eb)))
              (else (expect-end-error nxt 'try))))))
        ((return)          (let ((t (peek-token s)))
                             (if (or (eqv? t #\newline) (closing-token? t))

src/julia-syntax.scm

@@ -1332,25 +1332,29 @@
   (let ((tryb   (cadr e))
         (var    (caddr e))
         (catchb (cadddr e)))
-    (cond ((length= e 5)
+    (cond ((and (length> e 4) (not (equal? (caddddr e) '(false))))
            (if (has-unmatched-symbolic-goto? tryb)
                (error "goto from a try/finally block is not permitted"))
-           (let ((finalb (cadddr (cdr e))))
+           (let ((finalb (caddddr e)))
              (expand-forms
               `(tryfinally
-                ,(if (not (equal? catchb '(false)))
-                     `(try ,tryb ,var ,catchb)
-                     `(scope-block ,tryb))
+                ,(if (and (equal? catchb '(false)) (length= e 5))
+                     `(scope-block ,tryb)
+                     `(try ,tryb ,var ,catchb (false) ,@(cdddddr e)))
                 (scope-block ,finalb)))))
-          ((length= e 4)
-           (expand-forms
-            (if (symbol-like? var)
-                `(trycatch (scope-block ,tryb)
-                           (scope-block
-                            (block (= ,var (the_exception))
-                                   ,catchb)))
-                `(trycatch (scope-block ,tryb)
-                           (scope-block ,catchb)))))
+          ((length> e 3)
+           (and (length> e 6) (error "invalid \"try\" form"))
+           (let ((elseb (if (length= e 6) (cdddddr e) '())))
+             (expand-forms
+               `(,(if (null? elseb) 'trycatch 'trycatchelse)
+                 (scope-block ,tryb)
+                 (scope-block
+                   ,(if (symbol-like? var)
+                        `(scope-block
+                          (block (= ,var (the_exception))
+                                 ,catchb))
+                        `(scope-block ,catchb)))
+                 ,@elseb))))
           (else
            (error "invalid \"try\" form")))))
 
@@ -3587,7 +3591,7 @@ f(x) = yt(x)
             ((eq? (car e) 'symboliclabel)
              (kill)
              #t)
-            ((memq (car e) '(if elseif trycatch tryfinally))
+            ((memq (car e) '(if elseif trycatch tryfinally trycatchelse))
              (let ((prev (table.clone live)))
                (if (eager-any (lambda (e) (begin0 (visit e)
                                                   (kill)))
@@ -3653,7 +3657,7 @@ f(x) = yt(x)
         (and cv (vinfo:asgn cv) (vinfo:capt cv)))))
 
 (define (toplevel-preserving? e)
-  (and (pair? e) (memq (car e) '(if elseif block trycatch tryfinally))))
+  (and (pair? e) (memq (car e) '(if elseif block trycatch tryfinally trycatchelse))))
 
 (define (map-cl-convert exprs fname lam namemap defined toplevel interp opaq)
   (if toplevel
@@ -4446,9 +4450,10 @@ f(x) = yt(x)
             ;; (= tok (enter L)) - push handler with catch block at label L, yielding token
             ;; (leave n) - pop N exception handlers
             ;; (pop_exception tok) - pop exception stack back to state of associated enter
-            ((trycatch tryfinally)
+            ((trycatch tryfinally trycatchelse)
              (let ((handler-token (make-ssavalue))
                    (catch (make-label))
+                   (els   (and (eq? (car e) 'trycatchelse) (make-label)))
                    (endl  (make-label))
                    (last-finally-handler finally-handler)
                    (finally           (if (eq? (car e) 'tryfinally) (new-mutable-var) #f))
@@ -4465,11 +4470,20 @@ f(x) = yt(x)
                  ;; handler block postfix
                  (if (and val v1) (emit-assignment val v1))
                  (if tail
-                     (begin (if v1 (emit-return v1))
+                     (begin (if els
+                                (begin (if (and (not val) v1) (emit v1))
+                                       (emit '(leave 1)))
+                                (if v1 (emit-return v1)))
                             (if (not finally) (set! endl #f)))
                      (begin (emit '(leave 1))
-                            (emit `(goto ,endl))))
+                            (emit `(goto ,(or els endl)))))
                  (set! handler-level (- handler-level 1))
+                 ;; emit else block
+                 (if els
+                     (begin (mark-label els)
+                            (let ((v3 (compile (cadddr e) break-labels value tail))) ;; emit else block code
+                              (if val (emit-assignment val v3)))
+                            (emit `(goto ,endl))))
                  ;; emit either catch or finally block
                  (mark-label catch)
                  (emit `(leave 1))

src/utils.scm

@@ -79,6 +79,7 @@
 
 (define (caddddr x) (car (cdr (cdr (cdr (cdr x))))))
 (define (cdddddr x) (cdr (cdr (cdr (cdr (cdr x))))))
+(define (cadddddr x) (car (cdddddr x)))
 
 (define (table.clone t)
   (let ((nt (table)))

test/syntax.jl

@@ -2983,3 +2983,76 @@ macro m42220()
 end
 @test @m42220()() isa Vector{Float64}
 @test @m42220()(Bool) isa Vector{Bool}
+
+@testset "try else" begin
+    fails(f) = try f() catch; true else false end
+    @test fails(error)
+    @test !fails(() -> 1 + 2)
+
+    @test_throws ParseError Meta.parse("try foo() else bar() end")
+    @test_throws ParseError Meta.parse("try foo() else bar() catch; baz() end")
+    @test_throws ParseError Meta.parse("try foo() catch; baz() finally foobar() else bar() end")
+    @test_throws ParseError Meta.parse("try foo() finally foobar() else bar() catch; baz() end")
+
+    err = try
+        try
+            1 + 2
+        catch
+        else
+            error("foo")
+        end
+    catch e
+        e
+    end
+    @test err == ErrorException("foo")
+
+    x = 0
+    err = try
+        try
+            1 + 2
+        catch
+        else
+            error("foo")
+        finally
+            x += 1
+        end
+    catch e
+        e
+    end
+    @test err == ErrorException("foo")
+    @test x == 1
+
+    x = 0
+    err = try
+        try
+            1 + 2
+        catch
+            5 + 6
+        else
+            3 + 4
+        finally
+            x += 1
+        end
+    catch e
+        e
+    end
+    @test err == 3 + 4
+    @test x == 1
+
+    x = 0
+    err = try
+        try
+            error()
+        catch
+            5 + 6
+        else
+            3 + 4
+        finally
+            x += 1
+        end
+    catch e
+        e
+    end
+    @test err == 5 + 6
+    @test x == 1
+end