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POX is a language inspired by Prolog and MUMPS.

POX is semi-functional. With the exception of global persistent variables (prefixed with a carat), all variables are scoped to the function.

Functions are declared and optionally defined.

A function declaration consists of a determinacy notation (used for memoization), a function signature (function name followed by a list of args), and a set of constraints that determine correctness. In the style of prolog, these constraints are a set of sequences of operations or predicates, any of which can succeed. The sequences may not share data, and the order in which they are executed is implementation dependent (they may in fact run in parallel).

A function definition is an expression that implements the function.

If a definition is not provided, the simplest implementation that resolves the constraints in the declaration will be used; otherwise, the definition will be tested against those constraints at runtime (unless such checking is disabled in the interpreter).

Example code:

// line comment

/* bounded comment */

dec det fib(x) := x<=1, return 1; x>1, return x + fib(x-1). // fibonnacci is determinate. The generated implementation for this will be fast enough, so we do not need to define it.

dec nondet err(x) := isString(x). // err/1 can only be called on strings
def err(x) := print("Error: "), print(x), fail(). // this definition prints an error message and then forces backtracking

dec nondet setName(x) := ^name:=x.

POX is processed in several passes:

  • The first pass removes comments and resolves syntactic sugar like infix operators (simplepox).
  • The second pass converts the new prefix-notation-format intermediate language into a forth-like suffix-notation stack language (reversepox) and builds in the standard library.
  • The third pass converts this stack language into a binary format (packedpox).

Here is the above example code translated into simplepox:

dec(true, 'fib', ('x'), (_alt(_seq(_assertGTE(1,x), _return(1), _seq(_assertGT(x,1), _return(_add(x, fib(sub(x,1))))))))).

dec(false, 'err', ('x'), (isString(x))).
def(err, ('x'), (_seq(print("Error: "), _seq(print(x), fail())))).

dec(false, 'setName', ('x'), (_unifyPersist("name", x))).

Here it is in reversepox (without stdlib definitions):

:dect fib/1 '_ANONSEQ1/1' '_ANONSEQ2/1' _alt/2 def[fib/1] cmp not if fail ;;
:deft fib/1 '_ANONSEQ1/1' '_ANONSEQ2/1' _alt/2 ;;
:deft _ANONSEQ1/1 dup 1 _assertGTE/2 dup if 1 swap not if fail ;;
:deft _ANONSEQ2/1 dup 1 swap _assertGT/2 dup not if fail if _ANONSEQ3 ;;
:deft _ANONSEQ3/1 dup dup 1 swap sub fib add ;;

:decn err/1 isString/1 not if fail ;;
:defn err/1 _ANONSEQ4/1 ;;
:defn _ANONSEQ4/1 'Error: ' print _ANONSEQ5/1 ;;
:defn _ANONSEQ5/1 print fail ;;

:decn setName/1 def[setName/1] 'name' _getPersist swap cmp not if fail ;;
:defn setName/1 'name' _setPersist ;;

Packedpox is a gzipped messagepack dictionary. Here is the JSON equivalent to that messagepack:

{
	"def":{
		"fib/1":{"det":true, "mem":{}, "code":["'_ANONSEQ1/1'", "'_ANONSEQ2/1'", "_alt/2"]},
		"err/1":{"det":false, "code":["_ANONSEQ4/1"]},
		"setName/1":{"det":false, "code":["'name'", "_setPersist"]},
		"_ANONSEQ1/1":{"det":true, "mem":{}, "code":["dup", "1", "_assertGTE/2", "dup", "if", "1", "swap", "not", "if", "fail"]},
		"_ANONSEQ2/1":{"det":true, "mem":{}, "code":["dup", "1", "swap", "_assertGT/2", "dup", "not", "if", "fail", "if", "_ANONSEQ3"]},
		"_ANONSEQ3/1":{"det":true, "mem":{}, "code":["dup", "dup", "1", "swap", "sub", "fib", "add"]},
		"_ANONSEQ4/1":{"det":false, "code":["'Error: '", "print", "_ANONSEQ5/1"]},
		"_ANONSEQ5/1":{"det":false, "code":["print", "fail"]}
	},
	"dec":{
		"fib/1":{"det":true, "mem":{}, "code":["'_ANONSEQ1/1'", "'_ANONSEQ2/1'", "_alt/2" "def[fib/1]" "cmp" "not" "if" "fail"]},
		"err/1":{"det":false, "code":["isString/1 not if fail"]},
		"setName/1":{"det":false", "code":["def[setName/1]", "'name', "_getPersist", "swap", "cmp", "not", "if", "fail"]}
	},
	"persist":{}
}

The 'mem' attribute of definitions and declarations stores memoized values for determinate functions. The key is some hashed version of a serialized form of the args.

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