Design of Error Type

[shubhamnarlawar]

Error type needs to be handled in below cases -

1. Assignment statement
2. Comparision statement
3. Typecast statement
4. Function call statement and return statement

For the below testcase,

public function main() {
    error simpleError = error("Simple error occurred");
}

Information available from the BIR is as follows -
A new error instruction consisting of -

1. Type
2. Lhs Operand
3. Message Operand
4. Cause Operand
5. Detail Operand

References - https://ballerina.io/learn/by-example/error-handling.html
https://ballerina.io/spec/lang/2020R1/#error-type-params

[shubhamnarlawar]

@ayonam - Please give your suggestions on designing error type.

[shubhamnarlawar]

To add support for -

1. creation of error object
2. assignment
3. conditional statement when assigned to union type

error | int err = foo();
if (err is error) {

}

[shubhamnarlawar]

Note - The error type has two components:
1 .reason — a string representing the reason for the error
2. detail — a mapping giving additional details about the error

Detail is basically record which consist more details about the error. We are not supporting record type at the moment, right?

Reference - https://medium.com/ballerina-techblog/ballerina-error-handling-part-i-d581f65c0f8d

[shubhamnarlawar]

@ayonam and I had a quick discussion about designing error type -

1. Declare error type the moment we get error instruction - i.e. INSTRUCTION_NEW_ERROR (enum value representing a new error)
2. To declare the type, declare the type of components present in error instruction.
3. Declare object error type in LLVM.
4. Box it using rust if required.
5. Instantiate the object.
6. Do alloca.
7. Assign field using getelementptr.
8. Do llvmstore.

[shubhamnarlawar]

A rust function needs to be generated as below -

error(param1, param2, param3)

param1 - "actual error string" - message operand
param2 - const load being typecasted to error type - cause operand
param3 - an immutable new map - detail operand

[shubhamnarlawar]

A rust function needs to be generated as below -

error(param1, param2, param3)

param1 - "actual error string" - message operand
param2 - const load being typecasted to error type - cause operand
param3 - an immutable new map - detail operand

This is inferred from text dump of BIR for sample program -

error simpleError = error("Simple error occurred");

    %0(RETURN) ();
    %0(RETURN) ();
    %1(LOCAL) error;
    %3(TEMP) string;
    %4(TEMP) error;
    %5(TEMP) ();
    %6(TEMP) map<anydata | readonly>;
    %7(TEMP) typeDesc<any | error>;
    %8(TEMP) map<anydata | readonly>;
    %9(TEMP) anydata;

    bb0 {
        %3 = ConstLoad hello error;
        %5 = ConstLoad 0;
        %4 = <error> %5;
;
        %8 = NewMap %7;
        %9 = cloneReadOnly(%8) -> bb1;
    }
    bb1 {
        %6 = <map<anydata | readonly>> %9;
        %1 = error error(%3, %4, %6);
        %0 = ConstLoad 0;
        GOTO bb3;
    }

[shubhamnarlawar]

In the instructions obtained from the BIR, I am getting a new instruction called - INSTRUCTION_KIND_NEW_TYPEDESC which is a type descriptor containing more details about the type which is a map (<map<anydata | readonly>> from above example).

Is this instruction similar to StructureInsn instruction?

[Kishanthan]

We need to support map<type>, anydata and readonly type to support a simple error construction. We need to break this in to sub tasks.

[shubhamnarlawar]

Design of Error Type

Overview

Model error type object in the Rust runtime and use an opaque pointer in the LLVM codegen side to hold it as its value.

Data model - C++/BIR representation

Extend Type class to TypeError which will contains error type structure details.

Codegen logic

Init logic

• A runtime lib API will be called to create a error type opaque pointer
  • It requires us to pass error type information such as error message as c_str().

Data model - Runtime (Rust)

• The Error type structure in the Rust implementation will be called BalError
• The members of BalError will contain the properties from the TypeError class

BIR parser updates

• Update ShapeCpInfo::read() to handle error types
• Update BIRReader::readInsn() to add support for instruction - NEW_ERROR

[shubhamnarlawar]

@ruvi-d - For storing the type information of error type, should I extend the existing type class or write a separate type class for error type? WDYT?

[ruvi-d]

I proposed an alternative approach under section "Data model - C++/BIR representation" in ballerina-platform/nballerina#194