Programs already need to have their input in the proper format in order to parse them. I'd like the mismatch error to be from the invocation of the program rath

Programs already need to have their input in the proper format in order to parse them. I'd like the mismatch error to be from the invocation of the program rather than halfway down the parsing. For this to work data structures would have to be nothing more than scalars, sequences and mappings without specific concrete types. Just like JSON, YAML, and the rest do it now.