Error Handling

Astra catches runtime problems with a structured when / thenblock, so a single bad line doesn't have to crash your whole program. You match on the specific error type and decide what happens next.

Why error handling

Without a when block, a runtime error stops execution and prints a diagnostic with the line number and error type:

whentest.astra

write x

terminal

[ASTRA-RUN-ERR] :: Line 1 :: UNDEFINED_VAR: x. (Check if the variable is declared)

⚠️ Without handling: the program exits immediately at the failing line. Wrapping it in when lets you recover instead.

when / then syntax

Put the risky code inside when:, and handle a specific failure with then ERROR_TYPE:. Close the whole block with ;.

whentest.astra

when:
    write x
then UNDEFINED_VAR:
    write "varaible not found"
;

Output:

varaible not found

The program kept running instead of stopping — x was never defined, Astra raised UNDEFINED_VAR, and the matching then block ran.

Catching multiple error types

Add as many then ERROR_TYPE: clauses as you need — each one only runs for its matching error:

whentest.astra

when:
    x = 10
    write x
    write x + TRUE
then UNDEFINED_VAR:
    write "varaible not found"
then INVALID_OPERATION:
    write "Boolean not in arithmatic"
;

Output:

10
Boolean not in arithmatic

x is defined this time, so write x succeeds and prints 10. The next line tries to add a boolean to a number, which raises INVALID_OPERATION — only that clause runs, the rest of the when block stops there.

Fallback then

A plain then:with no error type catches anything not matched by an earlier clause — useful as a catch-all so unexpected errors don't crash the program:

fallback.astra

when:
    write undefined_var
then UNDEFINED_VAR:
    write "Variable not found!"
then:
    write "Something went wrong"
;

💡 Tip: Order matters. List specific error types first, and put the fallback then: last so it only fires when nothing else matched.

Common error types

Error type	Raised when
UNDEFINED_VAR	A variable is used before it's declared.
INVALID_OPERATION	An operation mixes incompatible types, like adding a boolean to a number.
TYPE_MISMATCH	A value doesn't match the type a function or chain field expects.
INDEX_OUT_OF_RANGE	A chain is accessed at an index that doesn't exist.

ℹ️ Note:Dividing by zero doesn't raise an error at all — it returns INFINITE and keeps running. See Getting Started for that example.

← PreviousModifiers Next →File I/O