Define a function with a custom warning message

Question

Is it possible to define a function with a custom warning message ?

For example,

In[1]=  f[x_,y_]:=x-y (* with a warning message when the result is negative*)

In[2]=  f[5,3]
Out[2]= 2

In[3]=  f[3,5]
Out[3]= -2
Negative : the result is negative

Also I don't want to see duplicate warning messages. For example,

In[4]=  {f[3,7],f[4,6],f[5,7]}
Out[3]= {-4,-2,-2}
Negative : the result is negative

It produced three negative numbers, but it produced a single warning message.

Answer 1

You may e.g. use $Line:

f[x_, y_] := (If[x < y && tmp =!= $Line, 
   Print["Warning: negative result."]];
  tmp = $Line; x - y )

With this :

{f[3, 7], f[4, 6], f[5, 7]}