Function declarations

Quick examples:

function orElse
Maybe<a> ::= l::Maybe<a> r::Maybe<a>
{
  return if l.isJust then l else r;
}

function performSubstitution
TypeExp ::= te::TypeExp s::Substitution
{
  te.substitution = s;
  return te.substituted;
}

Functions declarations give a name for the function, and a signature in the same format as productions, except that the left-hand side is not named. Functions must also contain a return statement.

Like the children of productions, function parameters may be supplied inherited attributes in the function body, and are considered implicitly decorated.

Passing undecorated types to a function, accessing an attribute, and getting errors about missing inherited attribute equations is a common mistake for new Silver programmers. Sometimes the function should have taken a reference (Decorated Expr) type instead of the undecorated (Expr) type.

All parameters are passed lazily. There is currently no strictness annotation.

aspect function driver
_ ::= _ _ _
{
  tasks <- [emitDocumentation(ast)];
}

Aspect functions are also possible, by prefixing the declaration with aspect. Aspect functions are forbidden from using return, their purpose is only to influence the value of “collection production attributes” (which is not only a mouthful, but misnomer inside a function, too!)

Aspecting functions is discouraged. Future changes may deprecate this feature.

A more concise function declaration can often be used in cases where a function simply returns an expression. Examples:

fun foldr b ::= f::(b ::= a b)  i::b  l::[a] =
  if null(l) then i else f(head(l), foldr(f, i, tail(l)));

fun lookup Eq a => Maybe<b> ::= lst::[(a, b)] key::a = 
  case lst of
    [] -> nothing()
  | h::t -> if fst(h) == key then just(snd(h)) else lookup (t, key)
  end;

fun getName
  attribute name i occurs on a => 
  Integer ::= inst::Decorated a with i = inst.name;

These functions do not require a decoration context, which makes the concise function representation more efficient than ordinary functions. Functions of this style are simply treated as global named lambda expressions.