Whitespace is an esoteric programming language developed by Edwin Brady and Chris Morris at the University of Durham (also developers of the Kaya and Idris programming languages). It was released on 1 April 2003 (April Fool's Day). Its name is a reference to whitespace characters. Unlike most programming languages, which ignore or assign little meaning to most whitespace characters, the Whitespace interpreter ignores any non-whitespace characters. Only spaces, tabs and linefeeds have meaning. A consequence of this property is that a Whitespace program can easily be contained within the whitespace characters of a program written in another language, except possibly in languages which depend on spaces for syntax validity such as Python, making the text a polyglot.
The language itself is an imperative stack-based language. The virtual machine on which the programs run has a stack and a heap. The programmer is free to push arbitrary-width integers onto the stack (currently there is no implementation of floating point numbers) and can also access the heap as a permanent store for variables and data structures.
Commands are composed of sequences of spaces, tab stops and linefeeds. For example, tab-space-space-space performs arithmetic addition of the top two elements on the stack. Data is represented in binary using spaces (0) and tabs (1), followed by a linefeed; thus, tab-space-space-tab-space-tab-tab-linefeed is the binary number 1001011, which is 75 in decimal. All other characters are ignored and thus can be used for comments.
The sign of a number is given by its first character, [Space] for positive and [Tab] for negative.
Code is written as an Instruction Modification Parameter (IMP) followed by the operation. The table below shows a list of all the IMPs in Whitespace.
Each IMP is followed by one operation defined for that IMP, and a parameter if needed. The list of operations supported in Whitespace is:
|[Space]||[Space]||Number||Push the number onto the stack|
|[Space]||[LF][Space]||-||Duplicate the top item on the stack|
|[Space]||[Tab][Space]||Number||Copy the nth item on the stack (given by the argument) onto the top of the stack|
|[Space]||[LF][Tab]||-||Swap the top two items on the stack|
|[Space]||[LF][LF]||-||Discard the top item on the stack|
|[Space]||[Tab][LF]||Number||Slide n items off the stack, keeping the top item|
|[Tab][Tab]||[Space]||-||Store in heap|
|[Tab][Tab]||[Tab]||-||Retrieve from heap|
|[LF]||[Space][Space]||Label||Mark a location in the program|
|[LF]||[Space][Tab]||Label||Call a subroutine|
|[LF]||[Space][LF]||Label||Jump to a label|
|[LF]||[Tab][Space]||Label||Jump to a label if the top of the stack is zero|
|[LF]||[Tab][Tab]||Label||Jump to a label if the top of the stack is negative|
|[LF]||[Tab][LF]||-||End a subroutine and transfer control back to the caller|
|[LF]||[LF][LF]||-||End the program|
|[Tab][LF]||[Space][Space]||-||Output the character at the top of the stack|
|[Tab][LF]||[Space][Tab]||-||Output the number at the top of the stack|
|[Tab][LF]||[Tab][Space]||-||Read a character and place it in the location given by the top of the stack|
|[Tab][LF]||[Tab][Tab]||-||Read a number and place it in the location given by the top of the stack|
The "copy" and "slide" operations were added in Whitespace 0.3 and may not be supported by all implementations.
The following is a commented Whitespace program that simply prints "Hello, world!", where each Space, Tab, or Linefeed character is preceded by the identifying comment "S", "T", or "L", respectively:
S S S T S S T S S S L T L S S S S S T T S S T S T L T L S S S S S T T S T T S S L T L S S S S S T T S T T S S L T L S S S S S T T S T T T T L T L S S S S S T S T T S S L T L S S S S S T S S S S S L T L S S S S S T T T S T T T L T L S S S S S T T S T T T T L T L S S S S S T T T S S T S L T L S S S S S T T S T T S S L T L S S S S S T T S S T S S L T L S S S S S T S S S S T L T L S S L L L
Note that when Whitespace source code is displayed in some browsers, the horizontal spacing produced by a tab character is not fixed, but depends on its location in the text relative to the next horizontal tab stop. Depending on the software, tab characters may also get replaced by the corresponding variable number of space characters.
- Polyglot, a program valid in more than one language
- Steganography, the technique of concealing a message within another message
- Off-side rule languages, where blocks are expressed by whitespace indentation
- Python, the best-known example of a language with syntactically significant whitespace
- Esoteric programming languages
- "Whitespace". Compsoc. Archived from the original on 18 June 2015. Retrieved 8 December 2015.
- Timothy (1 April 2003). "New Whitespace-Only Programming Language". Slashdot. Retrieved 23 January 2014.
- Stroustrup, Bjarne. "Generalizing Overloading for C++2000" (PDF). Florham Park, NJ, USA: AT&T Labs. Retrieved 23 January 2014.
- Official website
- Release announcement on Slashdot
- The Whitespace Corpus A collection of interpreters, compilers, and programs for Whitespace
- Collection of Whitespace interpreters in various script languages
- Acme::Bleach A Perl module that rewrites the body of your module to a whitespace-only encoding ("for really clean programs").