1+

1+, initially conceived by Jeremy Tan (User:Parcly Taxel) in 2012 (but with the final specification and interpreter written in 2014), is an esoteric programming language where 1 is the only literal. It is mostly inspired by FALSE, Piet and Wang's B-machine.

Commands and syntax
1+ operates only on an initially empty stack of non-negative integers (negative integers cannot be represented, and indeed ensure that the jumps work; see below). Twelve of the language's thirteen commands do something to the stack:
 * 1 pushes the number 1 onto the stack. This is the only literal; all other numbers are constructed with operators like INTERCAL does. Unlike INTERCAL, however, this restriction is derived from the concept of integer complexity, the minimum number of 1's needed to make a number n with only additions and multiplications.
 * + pops the top two numbers and pushes their sum.
 * * is the same as + but pushes the product instead.
 * " duplicates the top number.
 * / rotates the stack upward, sending the top number to the bottom.
 * \ is the inverse of /.
 * ^ swaps the top two numbers.
 * < pops the top two numbers and pushes 0 if the top number is less than the second-top number, 1 otherwise. This is the only conditional in 1+.
 * . and , read input as a number or Unicode character respectively and push that on.
 * : and ; are corresponding commands for output, popping the top value and printing it as a number or Unicode character respectively.

# is the only control structure in 1+: it pops the top value n and sends program flow to immediately after the nth # in the current line of execution (where numbering starts from 0).

Subroutines are defined as (name|symbols) and are called as (name); they can call other subroutines or themselves (thereby implementing recursion) and the point where they are defined causes their execution too. They are separate lines of execution, so the numbering of #'s in them starts from 0. They are positioned within the overall program such that they never depend on a subroutine that is defined later on. Comments are placed in square brackets ([ ]).

Hello world program
11+"""1+"****"; [H] 111++""**1+(D|/"\"/^\)1++; [e] (D)11+""**++"";; [ll] 111+++"; [o] /"11+"""***+; [,] "11+"*+; [ ] "111++"/*\+; [W] \"; [o] 111+++; [r] (D)11+""**++; [l] +; [d] 11+""""****1+; [!] 111++"*1+; [\n]

External resources

 * Tumblr mirror of the specification
 * Interpreter for the language in Python