Shell: Add runtime errors and implement break/continue

Such errors are raised when SyntaxError nodes are executed, and are also
used for internal control flow.
The 'break' and 'continue' commands are currently only allowed inside
for loops, and outside function bodies.

This also adds a 'loop' keyword for infinite loops.

1 files changed, 35 insertions, 6 deletions

diff --git a/Base/usr/share/man/man5/Shell.md b/Base/usr/share/man/man5/Shell.md
index 3e94058914..77f84300da 100644
--- a/Base/usr/share/man/man5/Shell.md
+++ b/Base/usr/share/man/man5/Shell.md
@@ -221,6 +221,28 @@ $ for * { mv $it 1-$it }
 $ for i in $(seq 1 100) { echo $i >> foo }
 ```
 
+##### Infinite Loops
+Infinite loops (as denoted by the keyword `loop`) can be used to repeat a block until the block runs `break`, or the loop terminates by external sources (interrupts, program exit, and terminating signals).
+
+The behaviour regarding SIGINT and other signals is the same as for loops (mentioned above).
+
+###### Examples
+```sh
+# Keep deleting a file
+loop {
+    rm -f foo
+}
+```
+
+###### Examples
+```sh
+# Iterate over every non-hidden file in the current directory, and prepend '1-' to its name.
+$ for * { mv $it 1-$it }
+
+# Iterate over a sequence and write each element to a file
+$ for i in $(seq 1 100) { echo $i >> foo }
+```
+
 ##### Subshells
 Subshells evaluate a given block in a new instance (fork) of the current shell process. to create a subshell, any valid shell code can be enclosed in braces.
 
@@ -299,7 +321,7 @@ sequence :: variable_decls? or_logical_sequence terminator sequence
           | variable_decls? function_decl (terminator sequence)?
           | variable_decls? terminator sequence
 
-function_decl :: identifier '(' (ws* identifier)* ')' ws* '{' toplevel '}'
+function_decl :: identifier '(' (ws* identifier)* ')' ws* '{' [!c] toplevel '}'
 
 or_logical_sequence :: and_logical_sequence '|' '|' and_logical_sequence
                      | and_logical_sequence
@@ -318,12 +340,19 @@ pipe_sequence :: command '|' pipe_sequence
                | control_structure '|' pipe_sequence
                | control_structure
 
-control_structure :: for_expr
-                   | if_expr
-                   | subshell
-                   | match_expr
+control_structure[c] :: for_expr
+                      | loop_expr
+                      | if_expr
+                      | subshell
+                      | match_expr
+                      | ?c: continuation_control
+
+continuation_control :: 'break'
+                      | 'continue'
+
+for_expr :: 'for' ws+ (identifier ' '+ 'in' ws*)? expression ws+ '{' [c] toplevel '}'
 
-for_expr :: 'for' ws+ (identifier ' '+ 'in' ws*)? expression ws+ '{' toplevel '}'
+loop_expr :: 'loop' ws* '{' [c] toplevel '}'
 
 if_expr :: 'if' ws+ or_logical_sequence ws+ '{' toplevel '}' else_clause?