MATLAB: Misunderstanding InputParser Validation Behavior

inputparserMATLABvalidation

I seem to be confused as to the proper implementation of the inputParser when it comes to input validation. Consider the following function:
 function out = testInputParser(varargin)
     ip = inputParser;
     ip.addParameter('x', [], @double);
     ip.parse(varargin{:});
     out = ip.Results.x;
 end
These cases work as expected:
 % Should pass:
 >> testInputParser('x', 123) 
 ans =
   123
 % Should fail:
 >> testInputParser('x', '123')
 Error using testInputParser (line 5)
 The value of 'x' is invalid. It must satisfy the function:
 double.
…But then why do these cases NOT work?
 % Expected to pass, but fails
 testInputParser('x', -123)
 Error using testInputParser (line 5)
 The value of 'x' is invalid. It must satisfy the function:
 double.
 % Expected to fail, but passes
 >> testInputParser('x', 'a')
 ans =
 a
I can avoid some of this heartburn by using @isnumeric for my input validator, but I also want to ensure I've got doubles and not, say, int16 values coming in.
So, this seems like strange behavior to me. What am I missing?

Best Answer

@double is not an input validation function: it just takes the input and converts to double datatype. Whatever test is being done internally appears to accept positive results from the validation function.
You should be using a validation function such as @(x) isa(x, 'double') or @isfloat if single precision is also acceptable.
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