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Smurf - Language Specification
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Smurf = String-based MURiel Forthoid

Smurf is a tarpit based on the self-propagation paradigm featured in Muriel.
The only native data type is the string, and operations are carried out on
strings in a forty manner.

(At first I was reluctant to call the language Smurf after I found that there
was already a humourous email in circulation regarding a fictional (I hope
so, anyway...) language called Smurf. However, a Google search for "smurf
programming language" revealed this link:
http://www.google.com/search?q=cache:everything2.com/index.pl%3Fnode%3DForth+smurf+programming+language
I can only assume that this is a sign from a divine entity that there is
indeed a connection between Smurfs and the Forth programming language, so I'm
certainly not going to argue.)

The Smurf environment consists of a program (a string of characters), a stack
on which strings are stored, and a variable store which behaves as in a
standard imperative language - string values are labelled with a name, which
is also a string. Any string can be used as a variable name, including the
empty string. All variable values are initially set to the empty string.

The program consists of a sequence of instructions, which may be separated
with whitespace. Whenever an instruction takes a parameter from the stack,
that parameter is removed from the stack.

"insert some profound text here"
  - Places the string on top of the stack (without the quotes). The string
    may include the following escape sequences:
    \n = newline
    \" = the " character
    \\ = the \ character

+ - concatenates the top two strings on the stack. The string pushed earlier
    appears earlier in the resulting string, eg
    "Zork" "mid" +
    would result in the string "Zorkmid" being placed on the stack.

i - takes a string from user input, and places it on the stack.

o - outputs the topmost string on the stack.

p - Pops a variable name from the stack, pops a value from the stack, and
    assigns that value to the variable name.

g - Pops a variable name from the stack, and pushes the value of the
    variable.

h - Pops a string from the stack, and pushes its head, ie the first
    character. This causes an error if used on the empty string.

t - Pops a string from the stack, and pushes its tail, ie all but the first
    character. This causes an error if used on the empty string.

q - "Quotifies" the string on top of the stack, so that it can be placed
    into a Smurf program as a literal string, eg
    Arthur "two-sheds" Jackson
    becomes
    "Arthur \"two-sheds\" Jackson"
    NOTE: This differs from the quotify operator in Muriel, because it adds
    the surrounding quotes as well.

x - Executes the string on top of the stack as a Smurf program. The stack
    and variable store are cleared before it is executed.

--------------------------------
smurf.pl - the Smurf interpreter
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Usage:
smurf.pl <smurf program file>

Error messages are in the form of They Might Be Giants quotes. (I apologise
for my lack of knowledge of the works of Bjork, but if anyone would care to
provide me with appropriate quotes, I'll happily incorporate a command line
option to enable Icelandic Pop Diva Mode in the next version.)

"It's hard to understand me from the language I use
There's no word in English for my style":
The interpreter encountered an unrecognised program instruction.

"When the indicator says you're out of gas
Should you continue driving anyway?":
The program attempted to pop a value from the stack when it was empty.

"Roll out that special head
This is our favourite one":
The h instruction was used on the empty string.

"I'm not done
And I won't be till my head falls off":
The t instruction was used on the empty string.

"I was just talking and someone interrupted
Or was it a loud explosion?":

The terminating " of a string expression was missing.

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Example Smurf programs
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1. HelloWorld.smu
"Hello World!"o

2. Quine.smu
"\"\"p\"\"gqo\"\"go"""p""gqo""go

3. Echo.smu
io "\"a\"p \"io\" \"a\"gq+ \"a\"g+ x" "a"p "io" "a"gq+ "a"g+ x