Allow setting of fallback values in a json.parse call using annotations

[ymatto]

Description

In cases where I want my script to accept external json, I may want to use json.parse not only only with annotations using optional types (as is currently supported), but set fallback values that I want for my own script execution. Having to do this outside of the json.parse statement is a bit laborious and potentially redundant.

Preferable solution

It would be preferable to have some way to specify fallback values directly in the annotations for the json.parse call so I can, in one place, fully describe my expected json structure as well as what values to use if the input fails to fit that structure.

Alternative solutions

Currently I can do things record-by-record like this:

let json.parse x = my_json_file
an_int = x.foo ?? 0

However, this becomes laborious when the json structure is more complex.

Also, using annotations in the json.parse call is redundant, like this:

let json.parse ( x : { foo : int? }? ) = my_json_file
an_int = x.foo ?? 0

This allows any json to be used as input and my fallback value will be used if anything is wrong with the input json such that my expected record isn't present (or is the wrong type). However I essentially have to specify my expected type structure twice.

Additional context

No response

[toots]

This is a great idea! As far as I can see the biggest complication is that the code defining the terms depends on the code defining the types so adding an optional term to a type creates a dependency loop that is annoying to work with.

[ymatto]

I think i see what you mean.

Also, you might notice I didn't actually propose new syntax for what I'm asking for because... I can't think of something good.

I suppose I posted this in hopes it would trigger a good idea from somebody else, but perhaps it's not so simple.

[toots]

For syntax I was thinking:

let json.parse ( x : { foo : int ?? 0 } ) = my_json_file

This would match what's being done after parsing..

[ymatto]

Ah yes of course, that would be very nice.