Local Variable Type Hints

[APickledWalrus]

Problem

Local variables are essentially accepted wherever they are used. This results in more potential mistakes and (some) strange syntax behavior that is hard to track. Detailed type information for variables helps in two cases:

1. Mismatched types

It's possible you use the wrong variable somewhere. Detailed type information can catch this and print a warning (or error). Consider:

set {_a} to 5
set {_b} to "some string"
... do stuff ...
set {_c} to {_a} in lowercase # oops i used the wrong variable

We can catch that {_a} in lowercase is likely to be invalid and alert the user.

2. Syntax accepting objects

Consider syntax like ExprArithmetic:

set {_a} to "some string"
set {_b} to 1 + {_a}

With detailed type information, we can now identify at parse time that this is an invalid operation, instead of failing at runtime and returning none.

Solution

With systems like ExecutionIntent and multiple return types, we are finally in a position to execute this system well. Type tracking for local variables has been implemented across syntax interacting with variables using a HintManager that is attached to the ParserInstance.

The testing script along with HintManager's documentation provide insight into how hints are managed.

Assuming I can get this stable enough (i.e., depend on the type hints and avoid Object), I would likely consider making this an error. Due to addons, we might need a way to escape the error (operating "unsafely"?).

While the current draft is (somewhat) stable, further testing is needed, especially with addons.

Testing Completed

I have added a basic type hints.sk test (more to come). I plan to implement JUnit testing for HintManager.

Supporting Information

Depends on:

• Return type accuracy and usage improvements #7891

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Completes:

• Local variable type hints. Helps identify local variable return types #3864
• Variables can be used as function arguments of the wrong type #1229 (needs testing, may not error properly in current draft)

Related:

• Fix Syntax Removal #7879 (syntax conflicts (due to variables) would benefit from type hints)