FindMinimum, NMinimize, etc. with external process

Question

I was hoping to be able to use MMA's minimization power to work with an external process. I want to minimize the function 'TryThisNumber[x_]' which sends the value of x to something external and gets back a result from that external object. I want to find the number which minimizes the returned result. But ... I can only talk to the external process using strings. So, assume I have a function SendNumberToProcess[m_String] which does the actual communication with the process, I try:

TryThisNumber[x_]:= ToExpression[SendNumberToProcess[ToString[x]]];

and then,

FindMinimum[{TryThisNumber[a],0<a<10},{a}];

The problem is that ToString[x] in the above is just evaluating to the symbol, a, not a numerical value, and thus I'm not actually able to send a number to the external process. Any ideas?

Update. Presumably the problem is that FindMinimum has the HoldAll attribute, but this attempt doesn't work either:

TryThisNumber[x_]:= ToExpression[SendNumberToProcess[ToString[Evaluate[x]]]];

Answer 1

Thanks to J.M. and george2079, this works:

TryThisNumber[x_?NumericQ] := ToExpression[SendNumberToProcess[ToString[x]]];