FindMinimum, NMinimize, etc. with external process [duplicate]

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]]]];


marked as duplicate by J. M. will be back soon♦Apr 11 '18 at 13:51

• Have you tried TryThisNumber[x_?NumericQ] := (* stuff *)? – J. M. will be back soon Sep 21 '12 at 16:26
• It seems you forgot the NumericQ[] in your new definition... – J. M. will be back soon Sep 21 '12 at 16:37
• "I also wanted to ask what exactly is going on with the ?NumericQ construct, or rather, why specifically does it solve this problem." - See this. In short, the routines will first attempt a symbolic analysis of your objective function. If the function you want to optimize cannot be subjected to such analysis, then you need the _?NumericQ in the definition. – J. M. will be back soon Sep 22 '12 at 8:41
TryThisNumber[x_?NumericQ] := ToExpression[SendNumberToProcess[ToString[x]]];