# Introduction This language reference manual will not cover the basics of programming, instead it’s a reference manual for the language. If you want to test out the language there is a [REPL](https://docs.halon.io/manual/cli.html#script-interpreter) available. ## Variables Variables may store values from expression (e.g. constants or function calls) or callable function objects such as anonymous functions and named function pointers. In HSL variables are prefixed with `$` followed by `[a-zA-Z_]+[a-zA-Z0-9_]*`. Variable names are case-sensitive. Some variables are read-only, hence they are not allowed to be assigned to (primarily pre-defined variables in contexts). Variables are assigned by value ([copy-on-write](http://en.wikipedia.org/wiki/Copy-on-write)). ```hsl $var = "foo"; $bar = $var; $bar = ""; // $var is still "foo" ``` > **Note** > > Variables in HSL are main/function scoped. However a variable needs to be created in all code paths before being used. [`isset()`](functions.html.md#isset) may test if a variable exists. ## Functions A lot of builtin functions are available in the [function library](functions.html.md). Functions are called by name followed by parentheses `()` with input parameters in between them. Function names are case-sensitive. The argument types must be supported by the function, otherwise an error will be raised. It’s also possible to create [user-defined functions](structures.html.md#user-function). ```hsl echo uptime(); echo length("Hello"); ``` > **Warning** > > Calling a function with too few or too many arguments will raise an error, either during compilation time or at runtime (using [argument unpacking](structures.html.md#argumentunpacking)). ## Comments Comments may be added to the code using two syntaxes, C-style “multi-line” comments and C++ “single-line” comments. ```hsl /* multi-line comment */ // single-line comment ```