# Ch26. Vimscript Variables And Scopes Before diving into Vimscript functions, let's learn about the different sources and scopes of Vim variables. ## Mutable And Immutable Variable You can assign a value to a variable in Vim with `let`: ``` let pancake = "pancake" ``` Later you can call that variable any time. ``` echo pancake " returns "pancake" ``` `let` is mutable, meaning you can change the value at any time in the future. ``` let pancake = "not waffles" echo pancake " returns "not waffles" ``` Notice that when you want to change the value of a set variable, you still need to use `let`. ``` let beverage = "milk" beverage = "orange juice" " throws an error ``` You can define an immutable variable with `const` (both Vim variable setters are similar to Javascript's `let` and `const`). ``` const waffle = "waffle" echo waffle "return waffle const waffle = "pancake" " returns error let waffle = "pancake" " returns error ``` Before learning native Vimscript variables, it's good to learn the different sources of Vim variables that you can use. There are three more "variables" you can use inside Vim expressions: environment variable, option variable, and register variable. ### Environment Variable Suppose that you have a `SHELL` environment variable already available in your terminal. To access it from Vim: ``` echo $SHELL " returns $SHELL value. In my case, it returns /bin/bash ``` ### Option variable You can access Vim options with `&` (options are the settings you access with `:set`). To see what background Vim uses, you can run: ``` echo &background " returns either "light" or "dark" ``` Alternatively, you can always run `set background?` to see the value of the `background` option. ### Register variable You can access Vim registers (Ch. 08) with `@`. Suppose the value "chocolate" is already saved to register `a`. To access it, you can use `@a`. You can also update it with `let`. ``` echo @a " returns chocolate let @a .= " donut" echo @a " returns "chocolate donut" ``` Now when you paste from register `a` (`"ap`), it will return "chocolate donut". The operator `.=` concatenates two strings. The expression `let @a .= " donut"` is the same as `let @a = @a . " donut"` ## Variable Scopes There are 9 variable scopes in Vim. You can recognize them from their prepended letter: ``` g: Global variable {nothing} Global variable b: Buffer-local variable w: Window-local variable t: Tab-local variable s: Sourced Vimscript variable l: Function-local variable a: Function formal parameter variable v: Built-in Vim variable ``` ### Global variable When you are declaring a "regular" variable: ``` let pancake = "pancake" ``` `pancake` is actually a global variable. When you define a global variable, you can call them from anywhere. Prepending `g:` to a variable also creates a global variable. ``` let g:waffle = "waffle" ``` Both `pancake` and `g:waffle` have the same scope. You can call each of them with either syntax. ``` echo pancake " returns "pancake" echo g:pancake "returns "pancake" echo waffle " returns "waffle" echo g:waffle " returns "waffle" ``` ### Buffer variable A variable preceded with `b:` is a buffer variable. A buffer variable is a variable that is local to the current buffer (Ch 02). If you have multiple buffers open, each buffer will have their own separate list of buffer variables. In buffer 1: ``` const b:donut = "chocolate donut" ``` In buffer 2: ``` const b:donut = "blueberry donut" ``` Each `b:donut` buffer variable lives inside buffers 1 and 2, respectively. On the side note, Vim has a *special* buffer variable `b:changedtick` that keeps track of all the changes done to the current buffer. 1. Run `echo b:changedtick` and note the number it returns.. 2. Make changes in Vim. 3. Run `echo b:changedtick` again and note the number it now returns. ### Window variable A variable preceded with `w:` is a window variable. It exists only in that window. In window 1: ``` const w:donut = "chocolate donut" ``` In window 2: ``` const w:donut = "raspberry donut" ``` On each window, you can call `echo w:donut` to get unique values. ### Tab variable A variable preceded with `t:` is a tab variable. It exists only in that tab. In tab 1: ``` const t:donut = "chocolate donut" ``` In tab 2: ``` const t:donut = "blackberry donut" ``` On each tab, you can call `echo t:donut` to get unique values. ### Script variable A variable preceded with `s:` is a script variable. These variables can only be accessed from inside scripts. If you have an arbitrary file `script.vim` and inside it you have: ``` let s:dozen = 12 function Consume() let s:dozen -= 1 echo s:dozen " is left" endfunction ``` Source the file with `:source dozen.vim`. Now call the `Consume` function: ``` :call Consume() " returns "11 is left" :call Consume() " returns "10 is left" :echo s:dozen " Undefined variable error ``` The `Consume` function can be called directly and it increments as expected. When you try to read `s:dozen` directly, Vim won't find it because you are out of scope. `s:dozen` script variable is only accessible from inside `script.vim`. Each time you source the `script.vim` file, it resets the `s:dozen` counter. If you are in the middle of decrementing `s:dozen` value and you run `:source dozen.vim`, the counter resets back to 12. This can be a problem for unsuspecting users. To fix this issue, refactor the code: ``` if !exists("s:dozen") let s:dozen = 12 endif function Consume() let s:dozen -= 1 echo s:dozen endfunction ``` ### Function-local and Function formal parameter variable Both the function-local variable (`l:`) and the function formal variable (`a:`) will be covered in the next chapter. ### Built-in Vim variables A variable prepended with `v:` is a special built-in Vim variable. You cannot define these variables. You have seen some of them already. For example: - `v:version` tells you what Vim version you are using. - `v:key` contains the current item value when iterating through a dictionary. - `v:val` contains the current item value when running a `map()` or `filter()` operation. - `v:true`, `v:false`, `v:null`, and `v:none` are special data types. There are other variables. For a list of Vim built-in variables, check out `:h vim-variable` or `:h v:`. ## Using Vim Variable Scopes The Smart Way Being able to quickly access environment, option, and register variables give you a broad flexibility to customize your editor and terminal environment. You also learned that Vim has 9 different Vimscript variable scopes, each existing under a certain constraints. You can take advantage of these unique variable types to write your own custom plugins. But before that, let's learn how to create functions!