Thursday, May 29, 2008

arc functions

So the long and the short of it is, we get arc to evaluate expressions. Atoms mostly evaluate to themselves: strings, characters, and numbers.

arc> "foo"
arc> 12

Symbols are also atoms, but have the special property that they may be defined to have some other value. Quoting a symbol means it doesn't get evaluated.

arc> 'bar
arc> (set bar "toto")
arc> bar

Lists are another story: when arc evaluates a list, it assumes the first item of the list is a function, and the remainder are arguments.

arc> (+ 1 2)
arc> (expt 9 2)

The first item is not necessarily a function: it might also be a reference to a macro, or in anarki, an object for whose type a function has been prepared via defcall. For another day. The first place in a list is usually referred to as "functional position". So, the procedure for evaluating (a b c d), is to evaluate each of a, b, c, and d (order is unspecified). a should evaluate to a function, which is then called with the values of b, c and d as arguments.

But, the object in functional position doesn't have to be a symbol pointing to a function. It could also be an inline, anonymous function.

arc> ((fn (a b) (+ a b)) 23 67)

Disadvantage: it's dead ugly, and totally unreadable. Nest a few of these and you have parenthesis soup. It turns out this is one of the fundamental organising principles of arc code. So why bother?

  • group expressions

  • declare local variables

  • iteration

  • all sorts of wonderful things

- all this in a neat and concise way, without namespace clutter or parameter hell. Except, we need macros to make it happen.

Macros: a future post.

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