Casting strings removes the length parameter?

[kesmit13]

I've been working with some casting issues while writing a backend for SingleStoreDB. The SingleStoreDB is based on MySQL so it uses the same drivers, but there are SQL differences. I was doing some work with casts and noticed that you couldn't cast doubles, which is true of older versions of MySQL as well. SingleStoreDB adds a casting operation (:>) that can cast doubles, but also casts strings differently than the cast function. If you don't specify a length, it can truncate numbers. For example, doing cast x :> string where x is a double value 1000 will return '1'. If you use cast x :> char(10) then you get the full value as expected. However, Ibis appears to throw away the length value when doing casts. I wondered if anyone knew the reasoning why?

[cpcloud]

Thanks for opening an issue.

This behavior is ... interesting.

Ibis throws away the length parameter, because up until this discussion there are no string APIs (or generic APIs like .cast that use strings) where the length matters. All of our backends handle variable length strings so it didn't seem worth it to have another string type.

I don't think this behavior is enough to justify adding a whole new type, but you can special case the ops.Cast translation and look at the operation's output_dtype attribute as well as the input dtypes and choose your desired cast code.

Would that work for you?

[kesmit13]

I'll have to investigate further, but if the output_dtype contains that information, that should be fine. I already override the cast function in our backend.