Lesson 1.10: Strings

The purpose of this lesson is to explain how to use the String sort in K to represent sequences of characters, and explain where to find additional information about builtin functions over strings.

The String Sort

In addition to the Int and Bool sorts covered in Lesson 1.6, K provides, among others, the String sort to represent sequences of characters. You can import this functionality via the STRING-SYNTAX module, which contains the syntax of string literals in K, and the STRING module, which contains all the functions that operate over the String type.

Strings in K are double-quoted. The following list of escape sequences is supported:

Escape Sequence Meaning
\" The literal character "
\\ The literal character \
\n The newline character (ASCII code 0x0a)
\r The carriage return character (ASCII code 0x0d)
\t The tab character (ASCII code 0x09)
\f The form feed character (ASCII code 0x0c)
\x00 \x followed by 2 hexadecimal digits indicates a code point between 0x00 and 0xFF
\u0000 \u followed by 4 hexadecimal digits indicates a code point between 0x0000 and 0xFFFF
\U00000000 \U followed by 8 hexadecimal digits indicates a code point between 0x000000 and 0x10FFFF

Please note that as of the current moment, K's unicode support is not fully complete, so you may run into errors using code points greater than 0xff.

As an example, you can construct a string literal containing the following block of text:

This is an example block of text.
Here is a quotation: "Hello world."
	This line is indented.

Like so:

"This is an example block of text.\nHere is a quotation: \"Hello world.\"\n\tThis line is indented.\n\xc1\xc9\xcd\xd3\xda\n"

Basic String Functions

The full list of functions provided for the String sort can be found in domains.md, but here we describe a few of the more basic ones.

String concatenation

The concatenation operator for strings is +String. For example, consider the following K rule that constructs a string from component parts (lesson-10.k):

module LESSON-10
  imports STRING

  syntax String ::= msg(String) [function]
  rule msg(S) => "The string you provided: " +String S +String "\nHave a nice day!"

Note that this operator is O(N), so repeated concatenations are inefficient. For information about efficient string concatenation, refer to Lesson 2.14.

String length

The function to return the length of a string is lengthString. For example, lengthString("foo") will return 3, and lengthString("") will return 0. The return value is the length of the string in code points.

Substring computation

The function to compute the substring of a string is substrString. It takes two string indices, starting from 0, and returns the substring within the range [start..end). It is only defined if end >= start, start >= 0, and end <= length of string. Here, for example, we return the first 5 characters of a string:

substrString(S, 0, 5)

Here we return all but the first 3 characters:

substrString(S, 3, lengthString(S))


  1. Write a function that takes a paragraph of text (i.e., a sequence of sentences, each ending in a period), and constructs a new (nonsense) sentence composed of the first word of each sentence, followed by a period. Do not worry about capitalization or periods within the sentence which do not end the sentence (e.g. "Dr."). You can assume that all whitespace within the paragraph are spaces. For more information about the functions over strings required to implement such a function, refer to domains.md.

Next lesson

Once you have completed the above exercises, you can continue to Lesson 1.11: Casting Terms.