It's not a bad idea, as text file formats are all strings anyway, to let the application do the conversion. It's the one with the domain knowledge anyway. Perhaps it could do with a companion library to specify the data format and emit parse errors as appropriate. But to keep that out of the syntax makes a lot more sense than the insanity that is yaml.

I've been using toml for my latest project and the one thing that really bitten me is that [1,2,3] is a valid array [1.1, 1.2, 1.3] is a valid array, but [1, 1.5, 2] isn't valid and throws an exception due to it having heterogeneous types.

This is something that was fixed in the forthcoming TOML 1.0 spec. Parsers that have been updated to support it will allow heterogeneous types in arrays, which your application can convert to a vector of floats or whatever collection type it uses internally.

Fantastic. So far that has been literally my only complaint about TOML.