To do real work in a program you have to be able to make decisions, iterate through collections of items and perform actions repeatedly. For this, you need control structures. Pony has control structures that will be familiar to programmers who have used most languages, such as
for, but in Pony, they work slightly differently.
The simplest control structure is the good old
if. It allows you to perform some action only when a condition is true. In Pony it looks like this:
if a > b then env.out.print("a is bigger") end
Can I use integers and pointers for the condition like I can in C? No. In Pony
if conditions must have type Bool, i.e. they are always true or false. If you want to test whether a number
a is not 0, then you need to explicitly say
a != 0. This restriction removes a whole category of potential bugs from Pony programs.
If you want some alternative code for when the condition fails just add an
if a > b then env.out.print("a is bigger") else env.out.print("a is not bigger") end
Often you want to test more than one condition in one go, giving you more than two possible outcomes. You can nest
if statements, but this quickly gets ugly:
if a == b then env.out.print("they are the same") else if a > b then env.out.print("a is bigger") else env.out.print("b bigger") end end
As an alternative Pony provides the
elseif keyword that combines an
else and an
if. This works the same as saying
else if in other languages and you can have as many
elseifs as you like for each
if a == b then env.out.print("they are the same") elseif a > b then env.out.print("a is bigger") else env.out.print("b bigger") end
Why can’t I just say “else if” like I do in C? Why the extra keyword? The relationship between
else in C, and other similar languages, is ambiguous. For example:
// C code if(a) if(b) printf("a and b\n"); else printf("not a\n");
Here it is not obvious whether the
else is an alternative to the first or the second
if. In fact here the
else relates to the
if(b) so our example contains a bug. Pony avoids this type of bug by handling
else differently and the need for
elseif comes out of that.
Control structures are expressions¶
The big difference for control structures between Pony and other languages is that in Pony everything is an expression. In languages like C++ and Java
if is a statement, not an expression. This means that you can’t have an
if inside an expression, there has to be a separate conditional operator ‘?’.
In Pony control flow statements like this are all expressions that hand back a value. Your
if statement hands you back a value. Your
for loop (which we’ll get to a bit later) hands you back a value.
This means you can use
if directly in a calculation:
x = 1 + if lots then 100 else 2 end
This will give x a value of either 3 or 101, depending on the variable lots.
else branches of an
if produce different types then the
if produces a union of the two.
var x: (String | Bool) = if friendly then "Hello" else false end
But what if my if doesn’t have an else? Any
else branch that doesn’t exist gives an implicit
var x: (String | None) = if friendly then "Hello" end
The same rules that apply to the value of an
if expression applies to loops as well. Let’s take a look at what a loop value would look like:
actor Main new create(env: Env) => var x: (String | None) = for name in ["Bob"; "Fred"; "Sarah"].values() do name end match x | let s: String => env.out.print("x is " + s) | None => env.out.print("x is None") end
This will give x the value “Sarah” as it is the last name in our list. If our loop has 0 iterations, then the value of its
else block will be the value of x. Or if there is no
else block, the value will be
actor Main new create(env: Env) => var x: (String | None) = for name in Array[String].values() do name end match x | let s: String => env.out.print("x is " + s) | None => env.out.print("x is None") end
Here x would be
You can also avoid needing
None at all by providing a default value for when the loop has 0 iterations by providing an
actor Main new create(env: Env) => var x: String = for name in Array[String].values() do name else "no names!" end env.out.print("x is " + s)
And finally, here the value of x is “no names!”
if allows you to choose what to do, but in order to do something more than once, you want a loop.
while loops are very similar to those in other languages. A condition expression is evaluated and if it’s true we execute the code inside the loop. When we’re done we evaluate the condition again and keep going until it’s false.
Here’s an example that prints out the numbers 1 to 10:
var count: U32 = 1 while count <= 10 do env.out.print(count.string()) count = count + 1 end
while is also an expression. The value returned is just the value of the expression inside the loop the last time we go round it. For this example that will be the value given by
count = count + 1 when count is incremented to 11. Since Pony assignments hand back the old value our
while loop will return 10.
But what if the condition evaluates to false the first time we try, then we don’t go round the loop at all? In Pony
while expressions can also have an
else block. In general, Pony
else blocks provide a value when the expression they are attached to doesn’t. A
while doesn’t have a value to give if the condition evaluates to false the first time, so the
else provides it instead.
So is this like an else block on a while loop in Python? No, this is very different. In Python, the
else is run when the
while completes. In Pony the
else is only run when the expression in the
Sometimes you want to stop part-way through a loop and give up altogether. Pony has the
break keyword for this and it is very similar to its counterpart in languages like C++, C#, and Python.
break immediately exits from the innermost loop it’s in. Since the loop has to return a value
break can take an expression. This is optional, and if it’s left out, the value from the
else block is returned.
Let’s have an example. Suppose you want to go through a list of names, looking for either “Jack” or “Jill”. If neither of those appear, you’ll just take the last name you’re given, and if you’re not given any names at all, you’ll use “Herbert”.
var name = while moreNames() do var name' = getName() if name' == "Jack" or name' == "Jill" then break name' end name' else "Herbert" end
So first we ask if there are any more names to get. If there are then we get a name and see if it’s “Jack” or “Jill”. If it is we’re done and we break out of the loop, handing back the name we’ve found. If not we try again.
name' appears at the end of the loop so that will be our value returned from the last iteration if neither “Jack” nor “Jill” is found.
else block provides our value of “Herbert” if there are no names available at all.
Can I break out of multiple, nested loops like the Java labeled break? No, Pony does not support that. If you need to break out of multiple loops you should probably refactor your code or use a worker function.
Sometimes you want to stop part-way through one loop iteration and move onto the next. Like other languages, Pony uses the
continue keyword for this.
continue stops executing the current iteration of the innermost loop it’s in and evaluates the condition ready for the next iteration.
continue is executed during the last iteration of the loop then we have no value to return from the loop. In this case, we use the loop’s
else expression to get a value. As with the
if expression, if no
else expression is provided,
None is returned.
Can I continue an outer, nested loop like the Java labeled continue? No, Pony does not support that. If you need to continue an outer loop you should probably refactor your code.
For iterating over a collection of items Pony uses the
for keyword. This is very similar to
foreach in C#,
in in Python and
for in Java when used with a collection. It is very different to
for in C and C++.
for loop iterates over a collection of items using an iterator. On each iteration, round the loop, we ask the iterator if there are any more elements to process, and if there are, we ask it for the next one.
For example, to print out all the strings in an array:
for name in ["Bob"; "Fred"; "Sarah"].values() do env.out.print(name) end
Note the call to
values() on the array — this is because the loop needs an iterator, not an array.
The iterator does not have to be of any particular type, but needs to provide the following methods:
fun has_next(): Bool fun next(): T?
where T is the type of the objects in the collection. You don’t need to worry about this unless you’re writing your own iterators. To use existing collections, such as those provided in the standard library, you can just use
for and it will all work. If you do write your own iterators, note that we use structural typing, so your iterator doesn’t need to declare that it provides any particular type.
You can think of the above example as being equivalent to:
let iterator = ["Bob"; "Fred"; "Sarah"].values() while iterator.has_next() do let name = iterator.next()? env.out.print(name) end
Note that the variable name is declared let, so you cannot assign to the control variable within the loop.
Can I use break and continue with for loops? Yes,
for loops can have
else expressions attached and can use
continue just as for
The final loop construct that Pony provides is
until. Here we evaluate the expression in the loop and then evaluate a condition expression to see if we’re done or we should go round again.
This is similar to
while in C++, C# and Java, except that the termination condition is reversed; i.e. those languages terminate the loop when the condition expression is false, but Pony terminates the loop when the condition expression is true.
The differences between
repeat in Pony are:
- We always go around the loop at least once with
repeat, whereas with
whilewe may not go round at all.
- The termination condition is reversed.
Suppose we’re trying to create something and we want to keep trying until it’s good enough:
actor Main new create(env: Env) => var counter = U64(1) repeat env.out.print("hello!") counter = counter + 1 until counter > 7 end
while loops, the value given by a
repeat loop is the value of the expression within the loop on the last iteration, and
continue can be used.
Since you always go round a repeat loop at least once, do you ever need to give it an else expression? Yes, you may need to. A
continue in the last iteration of a
repeat loop needs to get a value from somewhere, and an
else expression is used for that.