Learn: Definitions

Being a concatenative language, min does not really need named parameters or variables: symbols just pop elements off the main stack in order, and that’s normally enough. There is however one small problem with the traditional concatenative paradigm; consider the following program for example:

 dup dup 
 "\.zip$" match 
 swap fsize 1000000 > and 
 swap mtime now 3600 - >

This program takes a single string corresponding to a file path and returns true if it’s a .zip file bigger than 1MB that was modified in the last hour. Sure, it is remarkable that no variables are needed for such a program, but it is not very readable: because no variables are used, it is often necessary to make copies of elements and push them to the end of the stack – that’s what the dup and swap are used for.

The good news is that you can use the define operator and the : sigil to define new symbols, and symbols can also be set to literals of course.

Consider the following program:

 :filepath
 filepath "\.zip$" match
 filepath fsize 1000000 >
 filepath mtime now 3600 - >
 and and

In this case, the filepath symbol is defined and then used on the following three lines, each of which defines a condition to be evaluated. The last line contains just two and symbols necessary to compare the three conditions.

Lexical scoping and binding

min, like many other programming languages, uses lexical scoping to resolve symbols.

Consider the following program:

 4 :a
 ( 
   a 3 + :a
   (
      a 1 + :a
      (a dup * :a) dequote
   ) dequote
 ) dequote

…What is the value of the symbol a after executing it?

Simple: 4. Every quotation defines its own scope, and in each scope a new variable called a is defined. In the innermost scope containing the quotation (a dup * :a) the value of a is set to 64, but this value is not propagated to the outer scopes. Note also that the value of a in the innermost scope is first retrieved from the outer scope (8).

If we want to change the value of the original a symbol defined in the outermost scope, we have to use the bind or its shorthand sigil @, so that the program becomes the following:

 4 :a ;First definition of the symbol a
 (
   a 3 + @a ;The value of a is updated to 7.
   (
     a 1 + @a ;The value of a is updated to 8
     (a dup * @a) dequote ;The value of a is now 64
   ) dequote
 ) dequote

quote-define and quote-bind

So far, we saw how to use the define and bind operator (or better, their shorthand sigils : and @) to define new symbols or bind values to existing ones.

Consider the following example:

 (1 2 3 4 5) :my-list
 my-list (dup *) map

If run the program above in min shell by pasting the first and then the second line in it, you’ll get an error similar to the following:

 (!) <repl>(1,19) [map]: Incorrect values found on the stack:
 - expected: {top} quot quot {bottom}
 - got:      {top} quot int {bottom}
     <repl>(1,19) in symbol: map

This error says that when the map operator was evaluated, there were incorrect values on the stack. Two quotations were expected, but instead a quotation and an integer were found. How did this happen?

Basically, when my-list was pushed on the stack, it pushed all its item on top of the stack. If you run get-stack, it will return the following list:

 (1 2 3 4 5 (dup *))

This happens because by default min assumes that when you define a quotation you want to define a new operator rather than a list. The following program works as expected, and it returns a list containing the squares of the first five integer numbers:

 (dup *) :square
 (1 2 3 4 5) (square) map

To avoid this behavior – i.e. whenever you want to define a list of items rather than an operator that will be immediately evaluated when pushed on the stack, you have to use the quote-define and the quote-bind or their respective sigils # and =:

 (1 2 3 4 5) #my-list
 my-list (dup *) map ;Returns (1 4 9 16 25) 

Sealing symbols

Finally, symbols can be sealed to prevent accidental updates or deletions. By default, all symbols defined in the core min modules are sealed, so the following code if run in min shell will result in an error:

 5 :quote

…because the symbol quote is already defined in the root scope. However, note that the following code will not return an error:

 (5 :quote quote dup *) -> ;returns 25

…because the quote symbol is only defined in the root scope and can therefore be redefined in child scopes.

If you want, you can seal your own symbols so that they may not be redefined using the bind operator or deleted using the delete.

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