Mouse is a stack-based interpreted programming language descended from MUSYS, an earlier language for a DEC PDP/8 with 4096 words of memory.
In Mouse: a Language for Microcomputers, Grogono defines the language and develops an intepreter step-by-step as he introduces each feature of the language.
The first few chapters cover the basics: postfix expressions, variables, control structures, pointers and macros. Later chapters include two complete Mouse interpreters, one in Pascal, the other in Z80 assembly language.
Here's a brief summary of the Mouse language. Instructions pop their operands from and push their result on the stack. ( before -- after ) shows the stack effects of each operation.
Maths / Logic
Mouse is looking pretty Forthlike so far! \ is the equivalent of MOD in Forth:
+- ( x y -- z ) calculate z, the sum of x+y-- ( x y -- z ) calculate z, the difference of x-y*- ( x y -- z ) calculate z, the product of x×y/- ( x y -- z ) calculate z, the quotient of x/y\- ( x y -- z ) calculate z, the remainder of x/y<- ( x y -- z ) if x<y then z is true, otherwise false=- ( x y -- z ) if x=y then z is true, otherwise false>- ( x y -- z ) if x>y then z is true, otherwise false
Input / Output
Only ? doesn't have an exact equivalent in Forth. ?' is the same as Forth's KEY. !' is the same as EMIT and ! is .:
?- ( -- x ) read a number x from the keyboard?'- ( -- x ) read a character x from the keyboard!- ( x -- ) display a number x!'- ( x -- ) display a character x"…"- ( -- ) display the quoted string
Peek and Poke
: is the equivalent of ! in Forth. . is the equivalent of @:
:- ( x addr -- ) store x in address addr.- ( addr -- x ) read x from address addr
Control Structures
[ … | … ] is similar to Forth's IF … ELSE … THEN.
( … ↑ … ) is similar to Forth's BEGIN … WHILE … REPEAT:
[- ( x -- ) if x is false, jump to the matching|or]|- ( -- ) jump to the matching](not always implemented)]- ( -- ) end a[ … | … ]structure(- ( -- ) start a loop↑- ( x -- ) exit loop if x is false (often rendered as^))- ( -- ) end loop, jump back to matching(~- ( -- ) the remainder of the line is a comment
Macro Definitions
#x;- ( -- ) call macro x$x- ( -- ) define macro x@- ( -- ) end macro definition%- ( x -- z ) access macro parameter
Macros are the Mouse equivalent of subroutines. A macro is defined by $x … @ and called with #x;. Parameters can be passed between the macro name and semicolon. For example #x,7,5,9; will pass the parameters 7, 5 and 9 to x.
A macro accesses it's parameters using %: 1% for the first, 2% for the second, etc. A parameter is evaluated every time it's accessed and can be almost any valid Mouse code.
A macro has 26 local variables, A to Z. 26 macro names are available, A to Z.
Example Code
Here are a few classic examples:
- Hello World (which recently celebrated it's 40th birthday)
- Fibonacci Numbers (the typical bad example of recursion)
- Greatest Common Divisor (a better example of recursion)
Hello, World
"Hello, World!"$
Displays the string Hello, World. The exclamation mark isn't printed. An exclamation in a string instructs the interpreter to print a line break. All Mouse programs end with $.
Fibonacci Numbers
$F
1% N: ~ store parameter in N
N. 2 < [ N. ] ~ if N < 2 then return N
N. 1 > [ #F, N. 1 - ; ~ otherwise calculate F(N-1)
#F, N. 2 - ; ~ | and F(N-2)
+ ] ~ | and return their sum
@
F calculates Fibonacci numbers using the recurrence relation Fn = Fn-1 + Fn-2 with F0 = 0, F1 =1. Note this is really slow, Ο(φn). Calculating F40 takes 6 minutes. There are better ways to calculate Fibonacci numbers!
Greatest Common Divisor
$G
1% X: 2% Y: ~ store parameters
X. Y. = [ X. ] ~ if X = Y then GCD = X
X. Y. > [ #G, X. Y. - , Y. ; ] ~ otherwise subtract the
X. Y. < [ #G, Y. X. - , X. ; ] ~ | smallest from the
@ ~ | largest and recurse
Another example of recursion. G calculates the GCD using Dijkstra's method.
Further Reading
-
Grogono, Peter
Mouse: A Language for Microcomputers.
New York: Petrocelli Books, 1983. - David G Simpson has published several interpreters for Mouse.