Saturday, 17 November 2012

MathP is not CL Standards Compliant

Bad news! MathP, my very own mathematics notation library for Common Lisp, breaks the ANSI CL standard. The problem is that unread-char cannot be called consecutively without calling something that will read a character (like read-char). From the CL Hyperspec page for unread-char:
It is an error to invoke unread-char twice consecutively on the same stream without an intervening call to read-char (or some other input operation which implicitly reads characters) on that stream

I used it to basically peek multiple characters ahead, like peek-char, but for multiple characters. This may be why MathP doesn't work in CLISP. That allows me to have arbitrary operators. For example, I could have the following as distinct operators:
  • * for multiplication
  • ** (perhaps for exponentiation)
  • .* (perhaps for element-wise multiplication, like in MATLAB)
  • *. (dot product?)
  • *.* (perhaps for element-wise exponentiation)
  • etc.

I thought I was being clever, but such a use contravenes the standard pretty clearly. So I'm interested in alternatives. Does anyone know if it's possible to do multiple unread-chars with other stream implementations? Also, can you read lisp from them with standard reading functions?


  1. You can unread-string in this way:
    Make a concatenated stream from a string stream and *standard-input* and set the *standard-input* to it.
    Then in a pertinent moment check by CONCATENATED-STREAM-STREAMS if your string stream has been depleted and reset *standard-input* not to have a concatenated stream of concatenated stream of ... in the *standard-input* variable with time.

  2. Thanks Goheeca, I'd overlooked concatenated streams. That's a little corner of the CLHS I hadn't absorbed... It looks perfect for this type of problem.