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Overview

Kitten is a minimalistic, dynamically typed, concatenative programming language intended primarily for Web development. (This is an in-progress implementation of that language.) Kitten is inspired by the Cat programming language. The Kitten compiler (kitten) compiles Kitten programs into C, which can be compiled and linked against the Kitten runtime library (libkitten) to produce a standalone executable. Thus a Kitten program can be built anywhere you can find a C compiler, making it ideal for shared hosts, where installing development tools may not be possible. Kitten will eventually come with a standard prelude of definitions to make your development life easier. To build the compiler, you need GHC and Parsec; the runtime, GCC. Just download the sources and run make with your fingers crossed. The sources also include a shell script named kittenc that you can use to compile a Kitten program meow.kitten into an executable named meow.

Kitten has three built-in types:

  • Integer: a signed 64-bit integral type.

  • Float: a double-precision floating-point number.

  • Quotation: a vector of boxed values, used to represent UTF-32 strings, heterogeneous arrays and structures, and anonymous functions.

The Kitten language itself is very simple:

<program>    ::= <term>*
<term>       ::= <integer> | <float> | <quotation> | <word> | <string>
               | <definition> | <import>
<integer>    ::= [0-9]+
<float>      ::= [0-9]+ '.' [0-9]+
<quotation>  ::= '[' <term>* ']'
<word>       ::= [A-Za-z_][0-9A-Za-z_]*
<string>     ::= '"' [^"]* '"'
<definition> ::= "define" <word> <quotation>
<import>     ::= "import" <string>

Comments are given in parentheses (()), and can be nested.

All terms essentially denote functions, and juxtaposition of terms denotes function composition. Thus all expressions are written in terms of data flow only, i.e., completely point-free.

There are only two special forms in the language: define, which introduces a new word definition, and import, which imports a Kitten file for compilation into the final executable.

Hello world:

"Hello world!\n" write

Hello user:

(It is customary to give a comment alongside definitions briefly describing
the expected inputs and outputs, of the form “inputs -- outputs”.)

define join ([a] [b] -- [a b])
  [compose]

define greet (name --)
  ["Hello, " swap "!\n" join join]

define prompt (prompt -- input)
  [write read_line]

"What is your name? " prompt greet

Definitions

The following definitions are provided by the Kitten runtime library.

Core Definitions

  • [A] apply Evaluates the quotation at the top of the stack.

  • [A] [B] compose Returns the composition of the two quotations at the top of the stack.

  • A dup Duplicates the value at the top of the stack.

  • A [B] [C] if Applies [B] if A is true (nonzero); otherwise, applies [C].

  • A pop Discards the top value on the stack.

  • A quote Returns [A].

  • A B swap Swaps the two topmost values on the stack.

Arithmetic and Conditional

  • A B add Returns the sum of A and B.

  • A B div Returns the quotient of A and B.

  • A B eq Returns whether A and B are equal.

  • A B ge Returns whether A is greater than or equal to B.

  • A B gt Returns whether A is greater than B.

  • A isf Returns whether A is a float.

  • A isi Returns whether A is an integer.

  • A isq Returns whether A is a quotation.

  • A B le Returns whether A is less than or equal to B.

  • A B lt Returns whether A is less than B.

  • A B mod Returns the modulus of A and B.

  • A B mul Returns the product of A and B.

  • A B ne Returns whether A is not equal to B.

  • A B sub Returns the difference of A and B.

I/O Definitions

  • A putc Writes A to standard output as a character.

  • A write Writes A to standard output as a formatted value.

Standard Library

The following definitions are provided by the Kitten standard library:

  • read_line Reads a line of text into a quotation.