Add casts and string to datetime conversion #27
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disallow string to bool casts
we always truncate, like in C
polars does not write them properly to the database
we do not guarantte equivalence here (e.g. 0.0 vs 0), but we guarantee that a sequence of casts Float64 -> String -> Float64 is the identity (up to some epsilon).
so that e.g. mssql can return DATETIME2 instead of DATETIME
we currently only guarantee this to work for datetimes of the form YYYY-MM-DD HH:MM:SS.MILLIS (yes, exactly six digits after the comma, which makes sense since we guarantee all operations on datetimes on millisecond precision)
we allow __eq__ between a dtype instance and a dtype class. When comparing with a class, const and vararg are ignored.
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We have casts for the following pairs of types now:
We do not consider string to datetime conversion a cast (polars also does not do this), but have
.str.to_datetime()and.str.to_date()methods instead. These will support a format parameter in the future. Currently they acceptYY-MM-DD HH:MM:SS:MILLISonly. For float to string, we only guarantee equivalence up to trailing zeros. Float to int conversion throws on inf / nan, except for Sqlite (it does not throw on itself, and transform cannot do this).