The world's first hat-oriented programming language

Hatter is an esoteric programming language. It inaugurates the novel paradigm of hat-oriented programming, in which all computation is represented as taking and dropping data from/into hats. Hats are entities consisting of an argument stack and descriptions of data movements (called magic) to be executed when data is taken or dropped from/into it. Hats can also have other stacks for internal use. Data is generally moved, not copied. A hat can be called recursively, but all instances of a hat share the same argument and internal stacks.

Here is an example hat. It computes the factorial of a non-negative integer (the only data type in the language) using the classical recursive algorithm:

hat fac:
  in @->[[horn->@1]->pred]->[[if<-\fac]<-\nop]->[apply<-pred]->[mul<-@1]->@
  out @->[horn->if]->[if<-1]->@

This code describes a hat called fac, with associated inmagic (run when data is dropped into the hat) and outmagic (run when data is taken from the hat). @ represents the argument stack; @1 is an internal stack. <- and -> move data between hats. Multiple movement operations can be strung together into a movement stream; however, each magic must consist of only one movement stream.

Hatter has no control structures. It does have an if hat, but it behaves like a function (the arguments are evaluated). Conditional execution is achieved through if and apply, which provides the ability of dynamically selecting a hat to move data from/into. \ is a compile-time operator to take the hatid of a hat, a handler that can be fed to apply to make it behave like the corresponding hat.

For more information, see below.


Copyright © 2011 Vítor Bujés Ubatuba De Araújo
The content of this site, unless otherwise specified, can be freely used, with or without modifications, provided that the author is mentioned, preferably with the URL of the original document.