The Less Basic

More about Structs

Mutable Fields

In the last chapter, you were taught how to define structs and create objects. But the structs you created were immutable. You couldn't change the objects at all after you created them. 

Here was our original definition of Dog

defstruct Dog :
   name: String
   breed: String

We can create a dog by calling the Dog function with a provided name and breed. But once created, you cannot change a dog's name or breed. Here's how to define Dog with a setter function for changing its name.

defstruct Dog :
   name: String with: (setter => set-name)
   breed: String

Now we can use the set-name function to change a dog's name.

val d = Dog("Shadow", "Golden Retriever")
println("I used to be called %_." % [name(d)])
set-name(d, "Sir Shadow the Wise")
println("But now I am called %_." % [name(d)])

prints out

I used to be called Shadow.
But now I am called Sir Shadow the Wise.

With the above definition of Dog, we can change a dog's name but not its breed. If we want to be able to change the breed as well then we need to similarly give it a setter function. 

The convention is to call the setter function the same name as the field it's setting but with a set- prefix. Follow this convention unless you have a good reason not to.

Providing Custom Printing Behaviour

We are used to using the print and println functions for printing things. Almost all of Stanza's core types can be printed using print. But print doesn't yet know how to print Dog objects. So the following

val d = Dog("Shadow", "Golden Retriever")
println("They call me %_." % [d])

prints out the fairly useless message

They call me [Unprintable Object].

Here is how to provide custom printing behaviour for Dog objects.

defmethod print (o:OutputStream, d:Dog) :
   print(o, "%_ the %_." % [name(d), breed(d)])

Now the same code

val d = Dog("Shadow", "Golden Retriever")
println("They call me %_." % [d])

prints out

They call me Shadow the Golden Retriever.

The defmethod keyword extends a defined multi with a new method. We'll learn what that all means later. This gives you small taste of Stanza's multimethod functionality and is the basis for Stanza's class-less object system. 

In the body of the print method

print(o, "%_ the %_." % [name(d), breed(d)])

be especially mindful of the o argument to print. This argument says to print the message to the o output stream.

The Match Expression

Now that you are familiar with a number of different types and know how to create objects of each one, you'll have to learn how to differentiate between them. 

Here's how to write a function that does different things depending on whether its argument is an integer or a string.

defn what-am-i (x) :
   match(x) :
      (i:Int) : println("I am %_. I am an integer." % [i])
      (s:String) : println("I am %_. I am a string." % [s])

If we call it with an integer

what-am-i(42)

then it prints out

I am 42. I am an integer.

But if we call it with a string

what-am-i("Timon")

then it prints out

I am Timon. I am a string.

If we call it with neither an integer or a string

what-am-i(false)

then the program crashes.

FATAL ERROR: No matching branch.
   at stanzaproject/lessbasic.stanza:5.9
   at stanzaproject/lessbasic.stanza:9.0

General Form

Here's the general form of a match expression.

match(argument expressions ...) :
   (argname:ArgType ...) : body
   ...

A match expression

  1. computes the result of evaluating all the argument expressions,
  2. then tests to see whether the results match the argument types indicated in the first branch.
  3. If the types match, then the branch argument names are bound to the results, and the branch body is evaluated. The result of the branch is the result of the entire match expression.
  4. If the types do not match, then the subsequent branch is tried. This continues either until a branch finally matches, or no branch matches and the program crashes.

Name Shadowing

The match expression branches each start a new scope, and the branch arguments are only visible from within that scope. To avoid confusing you we gave new names (i and s) to the branch arguments in our example

defn what-am-i (x) :
   match(x) :
      (i:Int) : println("I am %_. I am an integer." % [i])
      (s:String) : println("I am %_. I am a string." % [s])

but you can really use any name for the branch arguments. In fact, it is common to use the same name as the value that you are matching on.

defn what-am-i (x) :
   match(x) :
      (x:Int) : println("I am %_. I am an integer." % [x])
      (x:String) : println("I am %_. I am a string." % [x])

Matching Multiple Arguments

The match expression supports matching against multiple arguments. Here's a function that returns different things depending on the types of both of its arguments.

defn differentiate (x, y) :
   match(x, y) :
      (x:Int, y:Int) : 0
      (x:Int, y:String) : 1
      (x:String, y:Int) : 2
      (x:String, y:String) : 3

If we call it with different combinations of integers and strings

println(differentiate(42, 42))
println(differentiate(42, "Timon"))
println(differentiate("Pumbaa", 42))
println(differentiate("Timon", "Pumbaa"))

it returns different results for each. The above prints out

0
1
2
3

Example: Cats and Dogs

Here's a definition of two structs, Cat and Dog

defstruct Dog : (name:String)
defstruct Cat : (name:String)

Here's the definition of a say-hi function that prints different messages depending on whether x is a Cat or Dog

defn say-hi (x) :
   match(x) :
      (x:Dog) : println("Woof says %_ the dog." % [name(x)])
      (x:Cat) : println("Meow says %_ the cat." % [name(x)])

Let's call it a few times. 

say-hi(Dog("Shadow"))
say-hi(Dog("Chance"))
say-hi(Cat("Sassy"))

prints out

Woof says Shadow the dog.
Woof says Chance the dog.
Meow says Sassy the cat.

Introducing Union Types

One problem with the say-hi function is that it allows us to pass obviously incorrect arguments to it, which crashes the program.

say-hi(42)

results in

FATAL ERROR: No matching branch.
   at stanzaproject/lessbasic.stanza:5.9
   at stanzaproject/lessbasic.stanza:9.0

This is because we didn't give x a type annotation

defn say-hi (x)

which we've said is equivalent to declaring it with the ? type.

defn say-hi (x:?)

The ? type, by definition, allows us to pass anything to it, so Stanza is doing what it should, even though it's not what we want.

We would like to give x a type annotation that prevents us from passing 42 to say-hi, but what should it be? It's neither Dog nor Cat because say-hi has to accept them both. The solution is to annotate say-hi to take eitherDog orCat

defn say-hi (x:Dog|Cat) :
   match(x) :
      (x:Dog) : println("Woof says %_ the dog." % [name(x)])
      (x:Cat) : println("Meow says %_ the cat." % [name(x)])

You can verify that calling say-hi with dogs and cats continue to work, but more importantly, that calling say-hi with 42 doesn't work. Attempting to compile

say-hi(42)

gives the error

Cannot call function say-hi of type Cat|Dog -> False with arguments of type (Int).

Cat|Dog is an example of a union type. Union types allow us to specify the concept of "either this type or that type". 

Branches with Unspecified Types

If you leave off the type annotation for an argument in a match expression branch, then Stanza will automatically infer it to have the same type as the match argument expression. The following

defn f (x:Int|String) :
   match(x) :
      (x:Int) : body
      (x) : body2

is equivalent to

defn f (x:Int|String) :
   match(x) :
      (x:Int) : body
      (x:Int|String) : body2

This is often used to provide a default branch to run when none of the preceeding branches match.

Revisiting the If Expression

Now that you've been introduced to the match expression, it's time to unveil the inner workings of the if expression. It turns out that the if expression is just a slightly decorated match expression.

if x < 4 :
   println("Do this")
else :
   println("Do that")

is completely equivalent to

match(x < 4) :
   (p:True) : println("Do this")
   (p:False) : println("Do that")

The if expression is an example of a simple macro. Macros are very powerful tools for simplifying the syntax of commonly used patterns. Stanza includes many constructs that are simply decorated versions of other constructs, each implemented as a macro. The defstruct statement is another example. Later, we'll learn how to write our own macros to provide custom syntax for common patterns.

The Is Expression

Often you simply want to determine whether an object is of a certain type. Here is a long-winded method for checking whether x is a Dog object or not.

val dog? = match(x) :
              (x:Dog) : true
              (x) : false

Because this operation is so common, Stanza provides a shorthand for it. The above can be rewritten equivalently as

val dog? = x is Dog

Here is the general form.

exp is Type

It first evaluates exp and then returns true if the result is of type Type. Otherwise it returns false. The is expression is another example of a convenience syntax implemented using a macro. As you've noticed by now, Stanza's core library makes heavy use of macros.

The negative form of the is expression is the is-not expression. The following determines whether x is not a type of Dog.

val not-dog? = x is-not Dog

Casts

Stanza's type system is designed primarily to be predictable, not necessarily smart. This means that, as the programmer, you will often be able to infer a more specific type for an object than Stanza. Here is an example.

defn meow (x:Cat) :
   println("Meow!!!")
  
defn f (x:Cat|Dog) :
   val catness = if x is Cat : 1 else : -1
   if catness > 0 :
      meow(x)

Attempting to compile the above gives the error

Cannot call function meow of type Cat -> False with arguments of type (Dog|Cat).

Stanza believes that x is a Dog|Cat, but from our reasoning, the only way that meow can be called is if catness is positive. And catness is only positive if x is a Cat. Therefore x must be a Cat in the call to meow and the code should be fine.

To force Stanza to accept x as a Cat, we can explicitly cast x.

defn f (x:Cat|Dog) :
   val catness = if x is Cat : 1 else : -1
   if catness > 0 :
      meow(x as Cat)

The cast tells Stanza to trust your assertion that x is indeed a Cat. If, for some reason, your reasoning is faulty and x turns out not to be a Cat, then the incorrect cast will cause the program to crash at that point.

Deep Casts

Stanza's cast mechanism is much more flexible than many other languages, and, in particular, supports the notion of a deep cast

Here is a function that takes an array of integers or strings, and replaces each string in the array with its length.

defn compute-lengths (xs:Array<Int|String>) :
   for i in 0 to length(xs) do :
      match(xs[i]) :
         (x:String) : xs[i] = length(x)
         (x:Int) : false

And here is a function that computes the sum of an array of integers.

defn sum-integers (xs:Array<Int>) :
   var sum = 0
   for i in 0 to length(xs) do :
      sum = sum + xs[i]
   sum   

Now, given an array containing both integers and strings, we want to first replace each string with its length, and then compute the sum of the integers in the array.

val xs = Array<Int|String>(4)
xs[0] = 42
xs[1] = 7
xs[2] = "Timon"
xs[3] = "Pumbaa"

compute-lengths(xs)
sum-integers(xs)

Attempting to compile the above gives us the error

Cannot call function sum-integers of type Array<Int> -> Int with arguments 
of type (Array<String|Int>).

Stanza is complaining that sum-integers requires an array of integers, so xs is an illegal argument as it might contain strings. 

But we know that xs will not contain any strings at that point because compute-lengths replaced all of them with their lengths. So we can use a cast to force Stanza to trust this assertion.

sum-integers(xs as Array<Int>)

With the above correction, the program now compiles and runs correctly.

Types as Contracts

The above was an example of a deep cast, because it wasn't a direct assertion about the type of xs, but about the types of the objects it contains. You might be wondering, then, what exactly does that cast do? Does it iterate through the array and check every element to see if it is an Int? You'll be relieved to hear that it does not. That would be hopelessly inefficient, and also impossible in general. 

To answer the question, let's investigate what the cast does in the case that we're wrong. Change the definition of compute-lengths to this.

defn compute-lengths (xs:Array<Int|String>) :
   for i in 0 to length(xs) - 1 do :
      match(xs[i]) :
         (x:String) : xs[i] = length(x)
         (x:Int) : false

It now forgets to check the last element. So even after the call to compute-lengthsxs still contains one last string ("Pumbaa") at the end, and thus our cast is incorrect.

Compile and run the program. It should crash with this error.

FATAL ERROR: Cannot cast value to type.
   at core/core.stanza:3062.16
   at stanzaprojects/lessbasic.stanza:13.18
   at core/core.stanza:2292.9
   at core/core.stanza:4042.16
   at stanzaprojects/lessbasic.stanza:12.28
   at stanzaprojects/lessbasic.stanza:23.0

The file position stanzaprojects/lessbasic.stanza:13.18 tells us that the error occurred in the reference to xs[i] in sum-integers. Stanza is saying that it was expecting xs[i] to be an Int because you promised that xs is an Array<Int>. But xs[i] is not an Int, and so your program is wrong.

In general, a value's type in Stanza does not directly say what it is. Instead, a value's type is a contract on how it should behave. Part of the contract for an Array<Int> is that it should only contain Int objects. The above program crashed as soon as Stanza determined that xs does not satisfy its contract. 

Operations on Strings

There are many useful operations on String objects available in the core library. We'll show a few of them here.

Length

Here's how to obtain the length of a string.

val s = "Hello World"
length(s)

Retrieve Character

Here's how to retrieve a given character in a string.

val s = "Hello World"
s[4]

The first character has index 0, and the last character is indexed one less than the length of the string. There is no function for setting the character in a string because strings are immutable in Stanza. 

Convert to String

Here's how to convert any object into a string.

to-string(42)

Append

Here's how to form a longer string from appending two strings together.

val s1 = "Hello "
val s2 = "World"
append(s1, s2)

Substring

Here's how to retrieve a range of characters within a string.

val str = "Hello World"
println(str[4 to 9])

prints out

o Wor

It's all the characters between index 4 (inclusive) and index 9 (exclusive) in the string.

If we wanted to include the ending index, then we can use the through keyword, just as we've learned from the previous chapter.

println(str[4 through 9])

prints out

o Worl

If we wanted to extract all characters from index 4 until the end of the string, we can use false as the ending index.

println(str[4 to false])

prints out

o World

Check out the reference documentation for a listing of operations supported by String objects.

Operations on Tuples

Tuples support a few additional operations for querying its properties.

Length

Here is how to retrieve the length of a tuple.

val t = [4, 42, "Hello"]
length(t)

Retrieve an Element

Here is how to retrieve an element in a tuple at a dynamically calculated index.

val t = [4, 42, "Hello"]
val i = 1 + 1
println(t[i])

prints out

Hello

Note that, in general, a dynamically calculated index is not known until the program actually runs. This means that Stanza does not try to determine a precise type for the result of t[i]. The resulting type of t[i] is the union of all the element types in the tuple.

Attempting to compile this

val t = [4, 42, "Hello"]
val x:Int = t[0 + 1]

results in the error

Cannot assign expression of type Int|String to value x with declared type Int.

The tuple t has type [Int, Int, String], and so an arbitrary element at an unknown index has type Int|String

To overcome this, you may explicitly cast the result to an Int yourself. 

val t = [4, 42, "Hello"]
val x:Int = t[0 + 1] as Int

Check out the reference documentation for a listing of operations supported by tuples.

Tuples of Unknown Length

The type [Int] is a tuple containing one integer, and the type [Int, Int] is a tuple containing two integers, et cetera. But what if we want to write a function that takes a tuple of any number of integers?

Here is a function that prints out every number in a tuple of integers.

defn print-tuple (t:Tuple<Int>) :
   for i in 0 to length(t) do :
      println(t[i])

The following

print-tuple([1, 2, 3])

prints out

1
2
3

But the following

print-tuple([1, "Timon"])

fails to compile with the error

Cannot call function print-tuple of type Tuple<Int> -> False with arguments
of type ([Int, String]).

In general, the type Tuple<Type>  represents a tuple of unknown length where each element type is of type Type

Packages

Thus far, all of your code has been contained in a single package. When your projects get larger, you'll start to feel the need to split up the entire program into smaller isolated components. In Stanza, you would do this by partitioning your program into multiple packages

Create a separate file called animals.stanza containing

defpackage animals :
   import core

defstruct Dog :
   name: String
defstruct Cat :
   name: String

defn sound (x:Dog|Cat) :
   match(x) :
      (x:Dog) : "woof"
      (x:Cat) : "meow"

The animals package contains all of our code for handling dogs and cats. It contains the struct definitions for Dog and Cat, as well as the sound function that returns the sound made by each animal.

Now create a file called mainprogram.stanza containing

defpackage animal-main :
   import core

defn main () :
   val d = Dog("Shadow")
   val c = Cat("Sassy")
   println("My dog %_ goes %_!" % [name(d), sound(d)])
   println("My cat %_ goes %_!" % [name(c), sound(c)])
  
main()

The animal-main package contains the main code of the program and it will use the animals package as a library. 

Importing Packages

Now compile both of your source files by typing in the terminal

stanza animals.stanza mainprogram.stanza -o animals

Oops! Something's wrong! Stanza reports these errors.

mainprogram.stanza:5.11: Could not resolve Dog.
mainprogram.stanza:6.11: Could not resolve Cat.
mainprogram.stanza:7.44: Could not resolve sound.
mainprogram.stanza:8.44: Could not resolve sound.

The problem is that our animal-main package never imported the animals package. Packages must be imported before they can be used. So change

defpackage animal-main :
   import core

to

defpackage animal-main :
   import core
   import animals

and try compiling again. Stanza still reports the same errors.

mainprogram.stanza:5.11: Could not resolve Dog.
mainprogram.stanza:6.11: Could not resolve Cat.
mainprogram.stanza:7.44: Could not resolve sound.
mainprogram.stanza:8.44: Could not resolve sound.

Public Visibility

What's going on? The problem now is that our animals package did not make any of its definitions public. By default, definitions are not visible from outside the package it is declared in. To make a definition visible, you must prefix the definition with the public keyword. 

Let's declare our Dog and Cat structs, and the sound function to be publicly visible.

defpackage animals :
   import core

public defstruct Dog :
   name: String
public defstruct Cat :
   name: String

public defn sound (x:Dog|Cat) :
   match(x) :
      (x:Dog) : "woof"
      (x:Cat) : "meow"

Now the program compiles successfully and prints out

My dog Shadow goes woof!
My cat Sassy goes meow!

Private Visibility

By default, all definitions are private to the package that they are defined in. There is no way to refer to a private definition from outside the package. This is a very powerful guarantee as it also means that there is no way for any outside code to depend upon the existence of a private definition. 

For example, suppose we rely on a helper function called dog? to help us define the sound function. 

defpackage animals :
   import core

public defstruct Dog :
   name: String
public defstruct Cat :
   name: String

defn dog? (x:Dog|Cat) :
   match(x) :
      (x:Dog) : true
      (x:Cat) : false

public defn sound (x:Dog|Cat) :
   if dog?(x) : "woof"
   else : "meow"

dog? is private to the animals package, so at any time in the future, if we wanted to rename dog? or remove it, we can safely do so without affecting other code.

Function Overloading

By this point, we've learned about arrays, tuples, strings, and how to retrieve the length of each of them.

val a = Array<Int>(4)
val b = "Timon and Pumbaa"
val c = [1, 2, 3, 4]
length(a) ;Retrieve length of a array
length(b) ;Retrieve length of a string
length(c) ;Retrieve length of a tuple

You simply call the length function. Here is what is happening behind the scenes. The core package actually contains many functions called length, but they differ in the type of the argument that they accept. 

defn length (x:Array) -> Int
defn length (x:String) -> Int
defn length (x:Tuple) -> Int

When you call length(a), Stanza automatically figures out which length function you are trying to call based on the type of its argument. a is an array, and so you're obviously trying to call the length function that accepts an Array. No other length function would be legal to call! Similarly, b is a string, so the call to length(b) is obviously a call to the length function that accepts a String. This is a feature called function overloading and is a key part of Stanza's object system. 

Functions can be overloaded based on the number of arguments that they take, and the types of each argument. Let's write our own overloaded function.

defstruct Dog
defstruct Tree
defstruct Captain

defn bark (d:Dog) -> False :
   println("Woof!")
defn bark (t:Tree) -> String :
   "Furrowed Cork"
defn bark (c:Captain) -> False :
   println("A teeeen-hut!")

Now let's try calling each of them. The following

val d = Dog()
val t = Tree()
val c = Captain()
bark(d)
println(bark(t))
bark(c)

prints out

Woof!
Furrowed Cork
A teeeen-hut!

Notice that the bark function for Tree returns a String, while the bark functions for Dog and Captain return False. There is no requirement for any of the bark functions to be related or aware of each other. They can even be declared in separate packages! 

Operator Mapping

In the previous chapter, you were introduced to the basic arithmetic operators. Here we'll show you a bit about how they work underneath. The following

val a = 13
val b = 24
a + b
a - b
a * b
a / b
a % b
(- a)

can be rewritten equivalently as

val a = 13
val b = 24
plus(a, b)
minus(a, b)
times(a, b)
divide(a, b)
modulo(a, b)
negate(a)

Thus you can see here that all operators in Stanza are simply syntactic shorthands for specific function calls. Here is a listing of what each operator expands to.

a + b     expands to   plus(a, b)
a - b     expands to   minus(a, b)
a * b     expands to   times(a, b)
a / b     expands to   divide(a, b)
a % b     expands to   modulo(a, b)
(- x)     expands to   negate(x)

a << b    expands to   shift-left(a, b)
a >> b    expands to   shift-right(a, b)
a >>> b   expands to   arithmetic-shift-right(a, b)
a & b     expands to   bit-and(a, b)
a | b     expands to   bit-or(a, b)
a ^ b     expands to   bit-xor(a, b)
(~ x)     expands to   bit-not(x)

a == b    expands to   equal?(a, b)
a != b    expands to   not-equal?(a, b)
a < b     expands to   less?(a, b)
a <= b    expands to   less-eq?(a, b)
a > b     expands to   greater?(a, b)
a >= b    expands to   greater-eq?(a, b)
not x     expands to   complement(x)

Operator Overloading

The benefit to mapping each operator to a function call is that you can very easily reuse these operators for your own objects. Here is an example struct definition for modeling points on the cartesian plane.

defstruct Point :
   x: Double
   y: Double

Next let's define a function called plus that can add together two Point objects.

defn plus (a:Point, b:Point) :
   Point(x(a) + x(b), y(a) + y(b))

Let's try out our function.

defn main () :
   val a = plus(Point(1.0,3.0), Point(4.0,5.0))
   val b = plus(a, Point(7.0,1.0))
   println("b is (%_, %_)" % [x(b), y(b)])

main()

The above prints out

b is (12.000000000000000, 9.000000000000000)

But, as mentioned, the + operator is a shorthand for calling the plus function. So our main function can be written more naturally as

defn main () :
   val a = Point(1.0,3.0) + Point(4.0,5.0)
   val b = a + Point(7.0,1.0)
   println("b is (%_, %_)" % [x(b), y(b)])

Get and Set

Two other operators that we have been using without being aware of it are the get and set operators. The following code

val a = Array<Int>(4)
a[0] = 42
a[0]

is equivalent to

val a = Array<Int>(4)
set(a, 0, 42)
get(a, 0)

Thus the a[i] form expands to calls to the get function.

a[i]         expands to   get(a, i)
a[i, j]      expands to   get(a, i, j)
a[i, j, k]   expands to   get(a, i, j, k)
etc ...

And the a[i] = v form expands to calls to the set function.

a[i] = v         expands to   set(a, i, v)
a[i, j] = v      expands to   set(a, i, j, v)
a[i, j, k] = v   expands to   set(a, i, j, k, v)
etc ...

Vectors

So far we've only called the library functions in the core package. The collections package contains commonly used datastructures useful for daily programming.

Here is a program that imports the collections package and creates and prints a Vector object.

defpackage mypackage :
   import core
   import collections

defn main () :
   val v = Vector<Int>()
   add(v, 1)
   add(v, 2)
   add(v, 3)
   println(v)

main()

It prints out

[1 2 3]

Vector object is similar to an array and represents a mutable collection of items where each item is associated with an integer index. However, whereas arrays are of fixed length, a vector can grow and shrink to accomodate more or less items. 

The type of the v vector in the example above is 

Vector<Int>

indicating that it is a vector for storing integers.

Here is how to add additional elements to the end of a vector.

add(v, 42)

Here is how to retrieve and remove the element at the end of the vector.

pop(v)

Identical to the case of arrays, here is how to retrieve the length of a vector, retrieve a value at a particular index, and assign a value to a particular index.

length(v) ;Retrieve a vector's length
v[0] = 42 ;Assign a value to index 0
v[0] ;Retrieve the value at index 0

HashTables

Hash tables are another commonly used datastructure in the collections package. A table associates a value object with a particular key object. It can be imagined as a two-column table (hence the name) where the left column is named keys and the right column is named values. Each entry in the table is recorded as a new row. The key object is recorded in the keys column, and its corresponding value object is recorded in the values column.

Here is how to create a HashTable where strings are used as keys, and integers are used as values.

val num-pets = HashTable<String,Int>()
num-pets["Luca"] = 2
num-pets["Patrick"] = 1
num-pets["Emmy"] = 3
println(num-pets)

The above prints out

["Patrick" => 1 "Luca" => 2 "Emmy" => 3]

Creation

The function

HashTable<String,Int>()

creates a new hash table that associates integer values with string keys. The type of the table created by the above function is

HashTable<String,Int>

which indicates that it is a hash table whose keys have type String and whose values have type Int.

Set

The calls to set

num-pets["Luca"] = 2
num-pets["Patrick"] = 1
num-pets["Emmy"] = 3

associates the value 2 with the key "Luca" in the table, the value 1 with "Patrick", and the value 3 with "Emmy". 

Get

Here's how to retrieve the value associated with a key.

println("Emmy has %_ pets." % [num-pets["Emmy"]])

which prints out

Emmy has 3 pets.

Does a Key Exist?

Attempting to retrieve the value in a table corresponding to a key that doesn't exist is a fatal error. Use the key? function to check whether a key exists in the table.

if key?(num-pets, "George") :
   println("George has %_ pets." % [num-pets["George"]])
else :
   println("I don't know how many pets George has.")

Default Values

A hash table can also be created with a default value. If a hash table has a default value, then this default value is returned when retrieving the corresponding value for a key that does not exist in the table. Change the definition of num-pets to

val num-pets = HashTable<String,Int>(0)

Now when we retrieve the number of pets owned by George,

println("George has %_ pets." % [num-pets["George"]])

it prints out

George has 0 pets.

KeyValue Pairs

KeyValue object represents an association between a key object and a value object. It can be created using the KeyValue function.

val kv = KeyValue(4, "Hello")

creates a KeyValue object that represents the mapping from the key 4 to the value "Hello". This is done very commonly, so Stanza also provides a convenient operator. The above can be written equivalently as

val kv = 4 => "Hello"

The type of the kv object created above is

KeyValue<Int,String>

which indicates that it represents an association between a key of type Int and a value of type String.

The key and the value objects in a KeyValue object can be retrieved using the key and value functions respectively.

key(kv) ;Retrieve the key
value(kv) ;Retrieve the value

For Loops over Sequences

Thus far you've only been shown how to use the for construct for simple counting loops. Here you'll see how the for construct generalizes to all types of collections.

The for loop can be used to iterate directly through the items of an array like so.

val xs = Array<Int>(4)
xs[0] = 2
xs[1] = 42
xs[2] = 7
xs[3] = 1

for x in xs do :
   println(x)

which prints out

2
42
7
1

General Form

Here is the general form.

for x in xs do :
   body

For each item in the collection xs, the for loop executes body once with x bound to the next item in the collection. In our example, xs contains the numbers 2427, and 1, and thus body is executed once each with x bound to 2427, and finally 1

Examples of Collections

We will more precisely specify what constitutes a collection later. For now, just accept that arrays, vectors, and tuples are collections, and strings are collections of characters. For example,

for c in "Timon" do :
   print("Next char is ")
   println(c)

prints out

Next char is T
Next char is i
Next char is m
Next char is o
Next char is n

And similarly,

for x in [1, 3, "Timon"] do :
   print("Next item is ")
   println(x)

prints out

Next item is 1
Next item is 3
Next item is Timon

In fact, Range objects are collections of integers, so the counting loops we saw before are actually just a special case of iterating through the items in a Range

val r = 0 to 4
for x in r do :
   print("Next number is ")
   println(x)

prints out

Next number is 0
Next number is 1
Next number is 2
Next number is 3

Tables are also collections, but they are collections of KeyValue objects, each representing one of the entries in the table. The following

val num-pets = HashTable<String,Int>()
num-pets["Luca"] = 2
num-pets["Patrick"] = 1
num-pets["Emmy"] = 3

for entry in num-pets do :
   println("%_ has %_ pets." % [key(entry), value(entry)])

prints out

Patrick has 1 pets.
Luca has 2 pets.
Emmy has 3 pets.

As you can see, Stanza's for construct is extremely powerful. In truth, the form shown here is still not the most general form of the for construct. We'll learn about that after we've covered first class functions.

Extended Example: Complex Number Package

In this extended example, we will implement a package for creating and performing arithmetic with complex numbers. 

The Complex Package

Create a file called complex.stanza with the following content.

defpackage complex :
   import core

public defstruct Cplx :
   real: Double
   imag: Double

This struct will be our representation for complex numbers. It is stored in cartesian form and has real and imaginary components. 

Printing Complex Numbers

To be able to print Cplx objects, we provide a custom print method.

defmethod print (o:OutputStream, x:Cplx) :
   if imag(x) >= 0.0 :
      print(o, "%_ + %_i" % [real(x), imag(x)])
   else :
      print(o, "%_ - %_i" % [real(x), (- imag(x))])

Main Driver

To test our program thus far, create a file called complexmain.stanza with the following content.

defpackage complex/main :
   import core
   import complex

defn main () :
   val a = Cplx(1.0, 5.0)
   val b = Cplx(3.0, -4.0)
   println(a)
   println(b)

main()

Compile and run the program by typing the following in the terminal.

stanza complex.stanza complexmain.stanza -o cplx
./cplx

It should print out

1.000000000000000 + 5.000000000000000i
3.000000000000000 - 4.000000000000000i

Great! So now we can create and print out complex numbers. If you're an electrical engineer, you may substitute i for j in the print method.

Arithmetic Operations

The next step is to implement the standard arithmetic operations for complex numbers. Pull out your old algebra textbooks and look up the formulas. Or pick up a pencil and derive them yourself. 

public defn plus (a:Cplx, b:Cplx) :
   Cplx(real(a) + real(b), imag(a) + imag(b))

public defn minus (a:Cplx, b:Cplx) :
   Cplx(real(a) - real(b), imag(a) - imag(b))

public defn times (a:Cplx, b:Cplx) :
   val x = real(a)
   val y = imag(a)
   val u = real(b)
   val v = imag(b)
   Cplx(x * u - y * v, x * v + y * u)

public defn divide (a:Cplx, b:Cplx) :
   val x = real(a)
   val y = imag(a)
   val u = real(b)
   val v = imag(b)
   val den = u * u + v * v
   Cplx((x * u + y * v) / den, (y * u - x * v) / den)

Let's test out our operators.

defn main () :
   val a = Cplx(1.0, 5.0)
   val b = Cplx(3.0, -4.0)
   println("(%_) + (%_) = %_" % [a, b, a + b])
   println("(%_) - (%_) = %_" % [a, b, a - b])
   println("(%_) * (%_) = %_" % [a, b, a * b])
   println("(%_) / (%_) = %_" % [a, b, a / b])

main()

The program prints out

(1.000000000000000 + 5.000000000000000i) + (3.000000000000000 - 4.000000000000000i)
    = 4.000000000000000 + 1.000000000000000i
(1.000000000000000 + 5.000000000000000i) - (3.000000000000000 - 4.000000000000000i)
    = -2.000000000000000 + 9.000000000000000i
(1.000000000000000 + 5.000000000000000i) * (3.000000000000000 - 4.000000000000000i)
    = 23.000000000000000 + 11.000000000000000i
(1.000000000000000 + 5.000000000000000i) / (3.000000000000000 - 4.000000000000000i)
    = -0.680000000000000 + 0.760000000000000i

which looks right to me!

Root Finding

Armed with our new complex number package, let's now put it to good use and solve an equation. We will use the Newton-Raphson method to solve the following equation.

x ^ 3 - 1 = 0

Here is our numerical solver which takes an initial guess, x0, and the number of iterations, num-iter, and performs num-iter number of Newton-Raphson iterations to find the root of the equation.

defn newton-raphson (x0:Cplx, num-iter:Int) :
   var xn = x0
   for i in 0 to num-iter do :
      xn = xn - (xn * xn * xn - Cplx(1.0,0.0)) / (Cplx(3.0,0.0) * xn * xn)
   xn   

Let's test it!

defn main () :
   println(newton-raphson(Cplx(1.0,1.0), 100))

The program prints out

1.000000000000000 + 0.000000000000000i

which is indeed one of the solutions to the equation! Fantastic!

Find all the Roots!

But according to the Fundamental Theorem of Algebra, the equation should have two more solutions. Different initial guesses will converge to different solutions so let's try a whole bunch of different guesses and try to find them all.

Here is a function that takes in a tuple of initial guesses and tries them all.

defn guess (xs:Tuple<Cplx>) :
   for x in xs do :
      val r = newton-raphson(x, 100)
      println("Initial guess %_ gave us solution %_." % [x, r])

And let's call it with a bunch of random guesses.

defn main () :
   guess([Cplx(1.0,1.0), Cplx(2.0,2.0), Cplx(-1.0,3.0), Cplx(-1.0,-1.0)])

The program prints out

Initial guess 1.000000000000000 + 1.000000000000000i gave 
   us solution 1.000000000000000 + 0.000000000000000i.
Initial guess 2.000000000000000 + 2.000000000000000i gave
   us solution 1.000000000000000 + 0.000000000000000i.
Initial guess -1.000000000000000 + 3.000000000000000i gave
   us solution -0.500000000000000 + 0.866025403784439i.
Initial guess -1.000000000000000 - 1.000000000000000i gave
   us solution -0.500000000000000 - 0.866025403784439i.

Thus the three solutions to the equation are 1-0.5 + 0.866i, and -0.5 - 0.866i. Problem solved!