Vesta SDL is a Functional Language

In a functional programming language, function calls do not have side-effects.  In other words, there are no global variables, and a function cannot change the value of variables in the calling scope.  A function's only inputs are its arguments, and a function's only output is its result value.

Most programmers are more familiar with imperative programming.  BASIC, Pascal, C, C++, Java, Perl, Python, and Tcl are all imperative programming languages (and there are many more).

Traps for Imperative Programmers

There are several aspects of Vesta SDL which may be confusing to people used to imperative programming languages:

• Statement blocks do not have side effects.  In other words, assignments made within a statement block do not leave that block.
• Function calls are a type of expression, but not a type of statement.  Something must be done with the return value of every function call (such as assigning it to a variable).
• The return/value statement can only ever appear at the end of the block of statements in the body of a function definition or at the end of a statement block expression.
• Assignments are a type of statement, but not a type of expression.
• if is a type of expression, but not a type of statement.

See the "Gotchas" page for more on these and other things that may cause confusion.

Syntactic Sugar

Vesta SDL has a few features which make it seem to be imperative in some ways.  Specifically:

• Assignment statements, as they appear to change the value of a variable.  (Really they introduce a new variable of the same name that shadows the old one.)
• Similarly, iteration is another method for introducing a new scope.  Each iteration creates a scope for the loop control variables that shadows those from previous iterations.  (Most functional languages lack iteration constructs, and recursion must be used instead.)

Language Implementation Details

The current implementation uses eager evalution. The main performance optimizations it uses are:

• Memoization, through the cache server.  (It's worth noting that this is a shared, persistent memoization mechanism.)
• Multi-threaded evaluation with the _par_map primitive function.