Loops in Magpie

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May 15, 2009 c-sharp code language magpie

I’m working on the loop syntax in Magpie right now, and I think I more or less have the plan figured out. Looping is a bit tricky in a language: there’s a ton of different ways to do it from C’s basic for (i = 0; i < 100; i++) to Lisp’s super-powerful (and super-complex) loop macro.

My goals for loops are the same as my goals for the language in general:

Add as few new keywords as possible.
Use function calls for as much of it as possible.
There must be no visible line between “baked-in” loop functions and user-defined ones.
The most common use cases should be the most terse.

The last point is especially pertinent, and is one of the guiding philosophies of the language: the things users do the most should be the most terse. Sort of like Huffman encoding for a language.

To accomplish that, I did a little archaeology: I looked at every place in the Magpie C# compiler where I’m using for or foreach. The results are:

There are 14 for loops and 40 foreach ones.
9 of the for loops are to iterate through two arrays in parallel.
3 of the for loops are to iterate backwards.
2 of the for loops are to assign to array elements.
1 of the for loops is to iterate a certain number of times.
None of the for or foreach loops reuse an existing variable for the current item.

From this, it’s pretty clear that I need to focus on iterating through collections (i.e. enumerators or generators), and assume that the user wants a new variable for the current item. Also, iterating through multiple collections simultaneously should be fairly easy to do. This leads me to adding just one new keyword, for:

for <var> <- <generator> do
    ...
end

In addition, multiple for clauses can be provided (but only one do) to iterate through multiple collections in parallel. I still need to work out the details, but I’m thinking that the above code will be syntactic sugar for:

// Evaluate the generator expression once.
def _generator <- Generate <generator>

// Advance to the first item.
_generator.MoveNext
while Not _generator.IsDone do
    def <var> <- _generator.Current
    ...
    _generator.MoveNext
end

Using that, the use cases I have can be solved by:

// Iterating through a collection.
for item <- someList do
    Print item
end

// Iterating multiple collections in lockstep.
for a <- someList1
for b <- someList2 do
    Print (a == b).String
end

// Iterating backwards.
for item <- Reverse someList
    Print item
end

// Assigning to array elements.
for index <- array.Indexes
    array.index <- 0
end

// Iterating a fixed number of times.
for i <- 50.Times
    Print i.String
end

// Iterating through a range.
for i <- Range (10, 30)
    Print i.String
end

In those examples, Reverse, Indexes, Times, and Range can all be simple library functions. There’s nothing magical about them. This may change when the rubber meets the road as I implement it, but it feels about right so far.