[HP PPL] Concatenation inside CAS(..) command does not work

[Le0ssa]

Hello, new Prime user here. I have the following program:

Code:


  local d, f, expression;
  if input({{f,[2]}}, "Derivate", "f(x)=") 
  then       
    expression:="diff(x^2,x)";
    d:= CAS(expression);
    return d; 
  end;

This program doesn´t work. It returns the string "diff(x^2,x")" instead of the actual differentiation result.

But if replace the above CAS call with:

Code:


    d:= CAS("diff(x^2,x)");

Then I get the proper diff output. Why?