Improve Error Handling Ergonomics

[smores56]

Design doc: version 1 and version 2

We want to improve the ergonomics of error handling in Roc. Below are listed the new tools we want to introduced, followed by example usages, and then a list of child issues each containing more details on implementing the new features.

The primary tool here is the new try keyword, which acts like Result.try, but is not quite the same as how ? currently desugars. It will pair well with the upcoming ! suffix behavior. It is used as a prefix in standard expressions, as well as in pipelines.

We want to pair it with a new ? operator, which acts like Result.mapErr. It can be used to purely or effectfully map over errors, meaning it can't just be syntax sugar. It is lower precedence than try and is used on the same line as an expression, like so:

fileContent = try File.read! "file.txt" ? FileReadErr

We also want to add ?? to mean Result.withDefault, which will work almost exactly like ? except that it can go on the next line in a pipelining expression chain.

confileFile = args.configPath ?? "~/.config/service.json"

Since ?? now means Result.withDefault, we should additionally update our default record syntax for types and values to use the same operator. That would look like:

Button a : {
    elements : List (Html a),
    onClick ?? Handler a,
}

renderButton = { elements, onClick ?? noOpHandler } -> ...

In all, we need the following features implemented:

• Move to the try Keyword for Error Propagation #7087
• Use ? for Mapping Result Errors #7088
• Use ?? for Providing Defaults for Results #7089
• Use ?? for Default Record Fields Types/Values #7090