Functions I

Functions in Wall are key-value pairs. They are roughly equivalent to dict in Python and Object in JavaScript, with the important caveat that they can have infinitely-large domains. In other words, all functions in Wall are treated as memoized functions, even if they are not actually implemented this way.

w> Paris France Berlin Germany =
w> { Paris: France, Berlin: Germany }
{ Paris: France, Berlin: Germany }

Invocation

Functions are no fun unless you can apply them to an argument and get a result. In Wall, this is done by separating the function and its argument with whitespace.

w> Paris France Berlin Germany =
w> { Paris: France, Berlin: Germany } Paris
France

Errors

Wall will issue a compile-time error if a function cannot be evaluated because it lacks (or might lack) an argument in its domain.

w> Paris France Berlin Germany Budapest =
w> { Paris: France, Berlin: Germany } Budapest
IncorrectDomainError. The function `{ Paris: France, Berlin: Germany }` does not contain `Budapest` in its domain.

Currying

There is no such thing in Wall as a function with more than one argument. Like in Haskell, all functions take one argument. They can, however, yield functions as values. This is commonly called Currying, named after Haskell Curry.

w> Left Right Up Down =
w> FinalDirection = {
    Left: { Right: Up, Left: Down },
    Right: { Left: Up, Right: Down }
}
w> FinalDirection Right Left
Up

You'll be seeing a lot of these

Functions are the only data structure available in Wall. They are inspired by Lua's tables, which are Lua's only data structure. That means that, when we see Sets and Lists later on, they will also be functions.