Error Handling, Deferred Execution

Error Handling

In MCScript, errors in the script and Java exceptions are collectively handled as ScriptError.

1 / 0 # line:1 / by zero

Integer.parseInt("a") # line:1 java.lang.NumberFormatException: For input string: "a"

throw

If you want to generate an error intentionally, use the throw statement.

throw "error"

ScriptError has no information other than the line number and error message.

try-catch

Errors thrown like Java can be handled using try-catch.

fun err() {
    throw "error"
}

try {
    err()
} catch e {
    e.print()
}

In this case, ErrorObject wrapped with ScriptError is passed to e. In addition to the lineNumber and message properties, ErrorObject has a print method that outputs the contents.

Deferred Execution

You can register the process in advance using the defer statement, and finally execute it with LIFO ordering.

defer Println("third")
Println("first")
defer Println("second")
/*
first
second
third
*/

If a defer statement is described in a function, it will be processed when the function is exited.

fun print() {
    defer Println("second")
    Println("first")
}

print()
defer Println("third")
/*
first
second
third
*/

The process is executed even if it is interrupted by an error.

fun err() {
    defer Println("first")
    throw "error"
}

try {
    defer Println("third")
    throw "error"
} catch e {
    defer Println("second")
    err()
}
/*
first
second
third
line:3 error
*/

Describes the process that should be executed last when the defer statement is called.

import java.io.FileInputStream;
import java.io.FileOutputStream;

fun copy(from, to) {
    try {
        input = FileInputStream(from)
        defer input.close()

        output = FileOutputStream(to)
        defer output.close()

        while (c = input.read()) != -1 {
            output.write(c)
        }

        output.flush()
    } catch e {
        e.print()
    }
}