Lesson 1.2: Basics of Functional K

The purpose of this lesson is to explain the basics of productions and rules in K. These are two types of K sentences. A K file consists of one or more requires or modules in K. Each module consists of one or more imports or sentences. For more information on requires, modules, and sentences, refer to Lesson 1.5. However, for the time being, just think of a module as a container for sentences, and don't worry about requires or imports just yet.

Our first K program

To start with, input the following program into your editor as file lesson-02-a.k:

module LESSON-02-A syntax Color ::= Yellow() | Blue() syntax Fruit ::= Banana() | Blueberry() syntax Color ::= colorOf(Fruit) [function] rule colorOf(Banana()) => Yellow() rule colorOf(Blueberry()) => Blue() endmodule

Save this file and then run:

kompile lesson-02-a.k

kompile is K's compiler. By default, it takes a program or specification written in K and compiles it into an interpreter for that input. Right now we are compiling a single file. A set of K files that are compiled together are called a K definition. We will cover multiple file K definitions later on. kompile will output a directory containing everything needed to execute programs and perform proofs using that definition. In this case, kompile will (by default) create the directory lesson-02-a-kompiled under the current directory.

Now, save the following input file in your editor as banana.color in the same directory as lesson-02-a.k:


We can now evaluate this K term by running (from the same directory):

krun banana.color

krun will use the interpreter generated by the first call to kompile to execute this program.

You will get the following output:

  Yellow ( ) ~> .

For now, don't worry about the <k>, </k>, or ~> . portions of this output file.

You can also execute small programs directly by specifying them on the command line instead of putting them in a file. For example, the same program above could also have been executed by running the following command:

krun -cPGM='colorOf(Banana())'

Now, let's look at what this definition and program did.

Productions, Constructors, and Functions

The first thing to realize is that this K definition contains 5 productions. Productions are introduced with the syntax keyword, followed by a sort, followed by the operator ::= followed by the definition of one or more productions themselves, separated by the | operator. There are different types of productions, but for now we only care about constructors and functions. Each declaration separated by the | operator is individually a single production, and the | symbol simply groups together productions that have the same sort. For example, we could equally have written an identical K definition lesson-02-b.k like so:

module LESSON-02-B syntax Color ::= Yellow() syntax Color ::= Blue() syntax Fruit ::= Banana() syntax Fruit ::= Blueberry() syntax Color ::= colorOf(Fruit) [function] rule colorOf(Banana()) => Yellow() rule colorOf(Blueberry()) => Blue() endmodule

You can try compiling and running lesson-02-b.k to see that it produces the same output as lesson-02-a.k:

kompile lesson-02-b.k
krun -cPGM='colorOf(Banana())' --definition 'lesson-02-b-kompiled'

where the --definition attribute points to the directory containing a compiled version of LESSON-02-B. Even the following definition is equivalent:

module LESSON-02-C syntax Color ::= Yellow() | Blue() | colorOf(Fruit) [function] syntax Fruit ::= Banana() | Blueberry() rule colorOf(Banana()) => Yellow() rule colorOf(Blueberry()) => Blue() endmodule

Each of these types of productions named above has the same underlying syntax, but context and attributes are used to distinguish between the different types. Tokens, brackets, lists, macros, aliases, and anywhere productions will be covered in a later lesson, but this lesson does introduce us to constructors and functions. Yellow(), Blue(), Banana(), and Blueberry() are constructors. You can think of a constructor like a constructor for an algebraic data type, if you're familiar with a functional language. The data type itself is the sort that appears on the left of the ::= operator. Sorts in K consist of uppercase identifiers.

Constructors can have arguments, but these ones do not. We will cover the syntax of productions in detail in the next lesson, but for now, you can write a production with no arguments as an uppercase or lowercase identifier followed by the () operator.

A function is distinguished from a constructor by the presence of the function attribute. Attributes appear in a comma separated list between square brackets after any sentence, including both productions and rules. Various attributes with built-in meanings exist in K and will be discussed throughout the tutorial.


Use krun to compute the return value of the colorOf function on a Blueberry().

Rules, Matching, and Variables

Functions in K are given definitions using rules. A rule begins with the rule keyword and contains at least one rewrite operator. The rewrite operator is represented by the syntax =>. The rewrite operator is one of the built-in productions in K, and we will discuss in more detail how it can be used in future lessons, but for now, you can think of a rule as consisting of a left-hand side and a right-hand side, separated by the rewrite operator. On the left-hand side is the name of the function and zero or more patterns corresponding to the parameters of the function. On the right-hand side is another pattern. The meaning of the rule is relatively simple, having defined these components. If the function is called with arguments that match the patterns on the left-hand side, then the return value of the function is the pattern on the right-hand side.

For example, in the above example, if the argument of the colorOf function is Banana(), then the return value of the function is Yellow().

So far we have introduced that a constructor is a type of pattern in K. We will introduce more complex patterns in later lessons, but there is one other type of basic pattern: the variable. A variable, syntactically, consists of an uppercase identifier. However, unlike a constructor, a variable will match any pattern with one exception: Two variables with the same name must match the same pattern.

Here is a more complex example (lesson-02-d.k):

module LESSON-02-D syntax Container ::= Jar(Fruit) syntax Fruit ::= Apple() | Pear() syntax Fruit ::= contentsOfJar(Container) [function] rule contentsOfJar(Jar(F)) => F endmodule

Here we see that Jar is a constructor with a single argument. You can write a production with multiple arguments by putting the sorts of the arguments in a comma-separated list inside the parentheses.

In this example, F is a variable. It will match either Apple() or Pear(). The return value of the function is created by substituting the matched values of all of the variables into the variables on the right-hand side of the rule.

To demonstrate, compile this definition and execute the following program with krun:


You will see when you run it that the program returns Apple(), because that is the pattern that was matched by F.


  1. Extend the definition in lesson-02-a.k with the addition of blackberries and kiwis. For simplicity, blackberries are black and kiwis are green. Then compile your definition and test that your additional fruits are correctly handled by the colorOf function.
  2. Create a new definition which defines an outfit as a multi-argument constructor consisting of a hat, shirt, pants, and shoes. Define a new sort, Boolean, with two constructors, true and false. Each of hat, shirt, pants, and shoes will have a single argument (a color), either black or white. Then define an outfitMatching function that will return true if all the pieces of the outfit are the same color. You do not need to define the case that returns false. Write some tests that your function behaves the way you expect.

Next lesson

Once you have completed the above exercises, you can continue to Lesson 1.3: BNF Syntax and Parser Generation.