03_Control_Flow.md

Chapter 3: Control Flow

Control flow is how your program makes decisions. It allows code to run only when certain conditions are met, or to repeat tasks.

1. Making Decisions (If/Else)

The valid syntax is if <condition>, followed by an indented block of code.

score = 85

if score > 50
    say "You passed!"
elif score == 50
    say "Just barely made it."
else
    say "Please try again."

Conditional Keywords

Keyword	Description
if	Execute block if condition is true
elif	Execute if previous conditions were false and this is true
else	Execute if all previous conditions were false
then	Optional keyword after condition (for readability)
do	Optional keyword after condition (for readability)

Natural Comparisons

You can use English words instead of math symbols if you prefer.

Symbol	English Phrase
==	is, is exactly, equals
!=	is not
>=	is at least, is more than or equal to
<=	is at most, is less than or equal to
>	is more than, is greater than
<	is less than

Example:

if the age is at least 18
    say "Adult"

if score is more than 100
    say "High score!"

if name is "Alice"
    say "Welcome, Alice!"

Unless

unless is the opposite of if. It runs only if the condition is no (false).

tired = no
unless tired
    say "Let's keep coding!"

This is equivalent to:

if not tired
    say "Let's keep coding!"

2. Pattern Matching (When)

If you have a long chain of if/else, when is cleaner. It looks like a "switch" statement in other languages.

day = "Monday"

when day
    is "Saturday"
        say "Weekend!"
    is "Sunday"
        say "Rest day."
    is "Monday"
        say "Back to work."
    otherwise
        say "Just another day."

The otherwise clause is optional and acts as a default case when no other conditions match.

3. Loops

Loops let you repeat code.

Repeat

The simplest loop. Runs a block a specific number of times.

repeat 3 times
    say "Hip Hip Hooray!"

You can also use it without times:

repeat 5
    say "Hello"

While

Runs while a condition is true.

count = 5
while count > 0
    say count
    count = count - 1

Natural language alternative:

doing as long as count > 0
    say count
    count -= 1

Until

Runs until a condition becomes true (opposite of while).

battery = 0
until battery == 100
    say "Charging..."
    battery += 10

For Loops

Iterates over a list or a range of numbers.

Range:

for i in range 1 5
    say i
# Prints: 1, 2, 3, 4, 5

Range with step:

for i in range 0 10 2
    say i
# Prints: 0, 2, 4, 6, 8, 10

Lists:

colors = ["red", "blue", "green"]
for color in colors
    say color

Dictionaries:

user = {"name": "Alice", "age": 30}
for key in user
    say key + ": " + str(user[key])

For Each

Alternative natural syntax:

for each item in items
    say item

Forever Loop

Sometimes you want a program to run indefinitely, for example, a server or a background watcher.

forever
    say "I will run until stopped manually."
    wait 1 second

Use stop or break to exit a forever loop when needed.

Loop Control Keywords

Keyword	Alias	Description
stop	break	Exits the loop immediately
skip	continue	Skips the rest of the current iteration

repeat 10 times
    if something_bad_happened
        stop  # Exit the loop

for i in range 1 10
    if i == 5
        skip  # Skip printing 5
    say i

4. Error Handling (Try/Catch)

Real world programs encounter errors. You can handle them gracefully using try, catch, and always (or finally).

try
    # Risky code
    result = 10 / 0
catch error
    # This runs if there is an error
    say in red "Oops! Something went wrong: " + error
always
    # This runs no matter what (success or error)
    say "Cleanup complete."

Error Handling Keywords

Keyword	Alias	Description
try		Begin a block that might throw an error
catch		Handle errors if they occur
always	finally	Always execute, regardless of error

Nested Try/Catch

try
    try
        risky_operation()
    catch inner_error
        say "Inner error handled"
catch outer_error
    say "Outer error caught"

5. Exiting the Program

To stop your program completely at any point, use exit.

if critical_system_failure
    say in red "Shutting down!"
    exit

6. Pass Statement

Sometimes you need a placeholder for code you haven't written yet. While ShellLite doesn't have a formal pass keyword, you can use an empty block or a comment:

if condition
    # TODO: implement this later

7. Conditional Expressions

ShellLite supports inline conditional expressions:

status = "adult" if age >= 18 else "minor"

8. Control Flow Summary

Statement	Purpose
if/elif/else	Conditional branching
unless	Negated conditional
when/is/otherwise	Pattern matching
repeat N times	Fixed iteration count
while	Loop while condition is true
until	Loop until condition becomes true
for X in Y	Iterate over collection
forever	Infinite loop
stop/break	Exit loop
skip/continue	Skip to next iteration
try/catch/always	Error handling
exit	Terminate program

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