It would be nice if the error about "Unable to find opcode entry for" would also provide what is expected. The information about what is expected is easily obtainable in the check_in_args() function, but this is called before the error message is printed. So in my hacked source the output reads like this:

Parameter 2 required type 'a' but got type 'S'
error:  Unable to find opcode entry for 'oscil' with matching argument types:
Found: a oscil cS

But it would be much nicer in the form of:

error:  Unable to find opcode entry for 'oscil' with matching argument types:
Found: a oscil cS
Parameter 2 required type 'a' but got type 'S'

I'm not sure what the best way to implement this is, but it's something that I think it would be very useful for newbies.