A string is basically a sequence of characters with a definite length. The type is the traditional C++ string type, and supports Unicode only by UTF-8 encoding.

Strings are 8 bit clean, meaning all 256 characters, including nul, may occur in them.

However string literals may not directly contain a nul. String literals are actually C nul terminated char strings which are lifted to C++ automatically.


The string type in Felix is the based on C++:

type string = "::std::basic_string<char>"
  requires Cxx_headers::string


There are two simple forms of string literals:

var x = "I am a string";
var y = 'I am a string too';

using double quote delimiters and single quote delimiters. These literals cannot exceed a single line. However you can concatenate them by juxtaposition:

var verse =
  "I am the first line,\n"
  'and I am the second.'

Notice the special encoding \n which inserts an end of line character into the string rather than a \ followed by an n. This is called an escape.

You can prevent escapes being translated with raw strings like this:

r"This \n is retained as two chars"

Only double quoted strings can be raw.

Felix also has triple quoted strings, which span line boundaries, and include end of lines in the literal:

which contains two end of line characters in the string, whilst this one:

Here is another
long string

has three end of lines (one after the first triple quote).


Use the len function:

var x = "hello";
var y =;


Strings can be concatenated with the infix + operator or just written next to each other, juxtaposition has a higher precedence than infix +.

var x = "middle";
var y = "Start" x + "end";

In this case, the first concatenation of x is done first, then the second one which appends “end”. The result is independent of the ordering because concatenation is associative, the run time performance, however, is not, because concatenation requires copying.

Substring Extraction

A substring of a string can be extracted using a slice with the notation shown:

var x = "Hello World";
var y = x.[3..7]; // 'lo Wo'


Select the n’th character:

var x = "Hello World";
var y = x.[1]; // e


Strings are totally ordered using standard lexicogrphical ordering and support the usual comparison operators.

Summary: String Comparisons

Operator Type Syntax Semantics
== string * string -> bool Infix Equality
!= string * string -> bool Infix Not Equal
<= string * string -> bool Infix Less or Equal
< string * string -> bool Infix Less
>= string * string -> bool Infix Greater or Equal
> string * string -> bool Infix Greater

Summary: Double Operations

Operator Type Syntax Semantics
len string -> size Prefix Length
+ string* string -> string Infix Concatenation
.[_] string * slice -> string Postfix Substring
.[_] string * int -> char Postfix Indexing