Error Handling

Mac reports errors at two stages: parse time (syntax errors) and runtime (logic errors). All errors print to stderr and cause the process to exit with code 1.

Parse Errors

Parse errors occur when the source code is syntactically invalid. The parser reports the line number and a description of what it expected.

[line 3] Error at '}': Expected ';' after expression.

Common causes:

• Missing semicolons after statements
• Unmatched braces, brackets, or parentheses
• Invalid token sequences (e.g. var = 5 without a name)

Runtime Errors

Runtime errors occur during execution when an operation is applied to the wrong type or an invalid state is reached. Each error prints Runtime Error: followed by a description.

Type Errors

// Arithmetic on non-numbers
"hello" - 5;
// Runtime Error: Operands must be numbers.

// String + non-string (only + between two strings or two numbers)
"hello" + 5;
// Runtime Error: Operands must be two numbers or two strings.

// Negating a non-number
-"text";
// Runtime Error: Operand must be a number.

Call Errors

// Calling a non-function
var x = 42;
x();
// Runtime Error: Can only call functions and classes.

// Wrong number of arguments
fun greet(name) { print "Hi, {name}"; }
greet("A", "B");
// Runtime Error: Expected 1 arguments but got 2.

Variable Errors

// Undefined variable
print x;
// Runtime Error: Undefined variable 'x'.

// Reassigning an immutable binding
val x = 10;
x = 20;
// Runtime Error: Cannot reassign 'val' binding 'x'.

Index and Property Errors

// Out-of-bounds array access
var a = [1, 2, 3];
print a[10];
// Runtime Error: Array index out of bounds.

// Non-numeric index
print a["key"];
// Runtime Error: Array index must be a number.

// Indexing a non-collection
var n = 42;
n[0];
// Runtime Error: Only arrays and maps support indexing.

// Missing map key
var m = { name: "Mac" };
print m["age"];
// Runtime Error: Undefined map key 'age'.

// Property access on a non-instance
var n = 42;
n.field;
// Runtime Error: Only instances and maps have properties.

Arithmetic Errors

// Division by zero
10 / 0;
// Runtime Error: Division by zero.

// Modulo by zero
10 % 0;
// Runtime Error: Modulo by zero.

Pipe and Compose Errors

// Piping into a non-function
5 |> 10;
// Runtime Error: Pipe target must be callable.

// Composing non-functions
val f = 5 >> 10;
// Runtime Error: Compose (>>) requires two callable values.

Iteration Errors

// Iterating over a non-collection
for (var x in 42) { print x; }
// Runtime Error: Can only iterate over arrays and maps.

Class Errors

// Non-class as superclass
var x = 42;
class Child < x {}
// Runtime Error: Superclass must be a class.

Exit Codes

When running a .mac file, the process exits with code 1 on the first error. In the REPL, errors are reported but execution continues for the next input.