On Thu, Dec 18 2014, Ulf Jasper wrote:

> It appears that the temperature value -1 °C ("M01") is causing the error
>
>     (metar-convert-temperature "M07") => (-7 . degC)
>     (metar-convert-temperature "M01") => error (wrong-type-argument stringp nil)
>
> I suspect that this is the result of `math-mul-symb-fancy' (which
> metar.el calls via calc-units) treating "-1" in a special way
>
>     (math-mul-symb-fancy -7 '(var degC var-degC)) => (* -7 (var degC var-degC))
>     (math-mul-symb-fancy -1 '(var degC var-degC)) => (neg (var degC var-degC))

There are problems in calc (Bug#19582) here.  There's also a small
problem in metar, viz., the string "-degC" is not parsed in the same way
as "-1 degC".

Since `metar-convert-unit' has (at least potentially) the same problem,
the attached patch does a little refactoring of those two functions,
where the actual conversion function from the calc library is passed as
an additional optional argument to `metar-convert-unit', so that
`metar-convert-temperature' reduces to just a call to the latter.

The actual bug is simply resolved by passing t as the last argument to
`math-convert-units' resp. `math-convert-temperature', so that these
functions return the number without a unit.  After all, we already know
which unit we get...

Please note that this patch assumes that the patch from (Bug#19582) has
been applied.