Category:Fexl

Fexl
This programming language may be used to instruct a computer to perform a task.

See Also:	Fexl on the HOPL

Listed below are all of the tasks on Rosetta Code which have been solved using Fexl.

Fexl is a functional programming language designed to be compact, efficient, and customizable in embedded environments. It has no keywords, so whenever you see a symbol in a Fexl program, that symbol always refers to a function.

A symbol may be defined within a Fexl program, or its definition may be left entirely open. The definitions of all open symbols are supplied by a context. A context is a function which supplies definitions for symbols. A context function itself is written in Fexl.

A Fexl program may therefore be resolved inside an arbitrary context, giving whatever meanings you like to all its open symbols. This makes it easy to wrap a Fexl program in an enhanced or restricted context. For example, in a web context you probably don't want "delete_file" to be defined at all, at least not in its normal sense. So you would you would resolve the program in a highly restricted context which only provides a handful of highly safe functions, omitting all the dangerous ones.

Ultimately the core functions of Fexl are written in the C programming language. To write a new core function named "X" within Fexl, you simply create an appropriate C routine named "fexl_X". It automatically becomes available as a built-in Fexl function.

For example, here is the C routine which implements the classic S combinator (fusion), where (S x y z) = (x z) (y z):

<lang C> value fexl_S(value f)

   {
   if (!f->L->L || !f->L->L->L) return f;
   return A( A(f->L->L->R, f->R), A(f->L->R, f->R) );
   }

</lang>

Fexl functions may have side effects -- after all, somebody's gotta do it. So for example here's the implementation of the char_put function, called as (char_get ch next), which prints the single character ch to stdout and then continues with next:

<lang C> value fexl_char_put(value f)

   {
   if (!f->L->L) return f;
   value x = f->L->R;
   if (!arg(type_long,x)) return f;

   putchar(get_long(x));
   return f->R;
   }

</lang>