Runtime Behavior and Errors

XE is intentionally strict about invalid operations. Earlier versions used silent fallbacks in a few places, but the current language runtime stops with explicit errors instead.

Two error categories

You will usually see one of two kinds of failures:

1. compiler errors
2. runtime errors

Compiler errors happen before code generation succeeds.

Runtime errors happen when the compiled program runs and reaches an invalid operation.

Compiler errors

Compiler errors include:

• undefined variables
• undefined functions
• wrong argument counts
• invalid indentation
• break outside loops
• continue outside loops
• return outside functions

The compiler prints:

• line and column
• the source line
• a caret pointing at the location

Example shape:

Error at line 1, column 7: undefined variable 'name'
print(name)
      ^

Runtime errors

Runtime errors currently cover cases like:

• division by zero
• modulo by zero
• invalid convert(...) operations
• invalid length(...) usage
• invalid index access
• invalid repeat counts
• iterating with for over unsupported values
• numeric comparisons on non-number values

Example:

print(convert("abc", "number"))

That stops with a runtime error instead of silently producing 0.

Indexing rules

Index access works on:

• lists
• text

items = [10, 20]
print(items[1])
print("XE"[0])

Current rules:

• the index must be a non-negative integer
• out-of-bounds access is a runtime error
• negative indexes are not supported

Conversion rules and failure cases

convert(...) is intentionally narrow.

Examples that work:

print(convert("42", "number"))
print(convert(1, "boolean"))
print(convert(true, "text"))

Examples that fail:

print(convert("abc", "number"))
print(convert([1, 2], "number"))

Current limitations worth knowing

XE is usable for small examples, but it still has deliberate limits:

• no list mutation
• no constants
• no modules or imports
• no file I/O or networking in the language
• no static type checker
• no closures

Knowing these limits makes it easier to write correct XE programs and to explain the language honestly.

Debugging advice

When something goes wrong:

1. start with xe compile file.xe and inspect the generated Rust if needed
2. reduce the program to the smallest failing example
3. check scope boundaries first, especially inside functions and blocks
4. check whether a conversion or index access is failing at runtime

Next steps

• Revisit Language Basics for a quicker overview
• Use Examples for runnable programs