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(This document was converted from the public-domain HTML Underload specification in The Esoteric File Archive.)


Underload is a stack-based programming language that works along similar lines to Muriel. Although not technically speaking a functional language, its evaluation operator ^ (which is the only form of flow control) makes programming in it functional in practice.

Reserved characters

The bracket and angle bracket characters []<> are reserved; if these are to appear anywhere in the program, they must be quoted by placing " before them. This also applies to the " character itself. Other characters must not be quoted with " (in particular, this means that Underload programs cannot output strings containing unmatched parentheses). Parentheses are moderately reserved, in that any Underload program must have matched parentheses to be legal.


  • ~ Swap the top two elements of the stack.
  • : Duplicate the top element of the stack.
  • ! Discard the top element of the stack.
  • * Concatenate the top element of the stack to the end of the second element of the stack.
  • ( Push everything between the ( and the matching ) on top of the stack.
  • ) Closes a ( command.
  • a Encloses the top element of the stack in a pair of parentheses.
  • ^ When the ^ command is called, it includes the top element of the stack into the program, immediately after the ^ command, ready to be run next.
  • S Output the top element of the stack, popping it.

Exceptional circumstances

Pretty much anything whose behaviour isn't specifically given here (for instance, running * on an empty stack) is an error.

Example programs

Hello, world!

(Hello, world!)S

Fibonacci sequence


Infinite loop




(The null program is also a quine. This program is more stack-based than functional.)