Developers

Introduction
Guide to your first Picker
Technical
- picker
- finders
- actions
- previewers

Introduction

So you want to develop your own picker and/or extension for telescope? Then you are in the right place! This file will first present an introduction on how to do this. After that, this document will present a technical explanation of pickers, finders, actions and the previewer. Should you now yet have an idea of the general telescope architecture and its components, it is first recommend to familiarize yourself with the architectural flow-chart that is provided in vim docs (:h telescope.nvim). You can find more information in specific help pages and we will probably move some of the technical stuff to our vim help docs in the future.

This guide is mainly for telescope so it will assume that you already have some knowledge of the Lua programming language. If not then you can find information for Lua here:

Lua 5.1 Manual
Getting started using Lua in Neovim

Guide to your first Picker

To guide you along the way to your first picker we will open an empty lua scratch file, in which we will develop the picker and run it each time using :luafile %. Later we will bundle this file as an extension.

Requires

The most important includes are the following modules:

local pickers = require "telescope.pickers"
local finders = require "telescope.finders"
local conf = require("telescope.config").values

pickers: main module which is used to create a new picker.
finders: provides interfaces to fill the picker with items.
config: values table which holds the user's configuration. So to make it easier we access this table directly in conf.

First Picker

We will now make the simplest color picker. (We will approach this example step by step, you will still need to have the previous requires section above this code.)

-- our picker function: colors
local colors = function(opts)
  opts = opts or {}
  pickers.new(opts, {
    prompt_title = "colors",
    finder = finders.new_table {
      results = { "red", "green", "blue" }
    },
    sorter = conf.generic_sorter(opts),
  }):find()
end

-- to execute the function
colors()

Running this code with :luafile % should open a telescope picker with the entries red, green, blue. Selecting a color and pressing enter will open a new file. In this case it's not what we want, so we will address this after explaining this snippet.

We will define a new function colors which accepts a table opts. This is good practice because now the user can change how telescope behaves by passing in their own opts table when calling colors.

For example the user can pass in a configuration in opts which allows them to change the theme used for the picker. To allow this, we make sure to pass the opts table as the first argument to pickers.new. The second argument is a table which defines the default behavior of the picker.

We have defined a prompt_title but this isn't required. This will default to use the text Prompt if not set.

finder is a required field that needs to be set to the result of a finders function. In this case we take new_table which allows us to define a static set of values, results, which is an array of elements, in this case our colors as strings. It doesn't have to be an array of strings, it can also be an array of tables. More on this later.

sorter on the other hand is not a required field but it's good practice to define it, because the default value will set it to empty(), meaning no sorter is attached and you can't filter the results. Good practice is to set the sorter to either conf.generic_sorter(opts) or conf.file_sorter(opts).

Setting it to a value from conf will respect the user's configuration, so if a user has set-up fzf-native as the sorter then this decision will be respected and the fzf-native sorter will be attached. It's also suggested to pass in opts here because the sorter could make use of it. As an example the fzf sorter can be configured to be case sensitive or insensitive. A user can set-up a default behavior and then alter this behavior with the opts table.

After the picker is defined you need to call find() to actually start the picker.

Replacing Actions

Now calling colors() will result in the opening of telescope with the values: red, green and blue. The default theme isn't optimal for this picker so we want to change it and thanks to the acceptance of opts we can. We will replace the last line with the following to open the picker with the dropdown theme.

colors(require("telescope.themes").get_dropdown{})

Now let's address the issue that selecting a color opens a new buffer. For that we need to replace the default select action. The benefit of replacing rather than mapping a new function to <CR> is that it will respect the user's configuration. So if a user has remapped select_default to another key then this decision will be respected and it works as expected for the user.

To make this work we need more requires at the top of the file.

local actions = require "telescope.actions"
local action_state = require "telescope.actions.state"

actions: holds all actions that can be mapped by a user. We also need it to access the default action so we can replace it. Also see :help telescope.actions
action_state: gives us a few utility functions we can use to get the current picker, current selection or current line. Also see :help telescope.actions.state

So let's replace the default action. For that we need to define a new key value pair in our table that we pass into pickers.new, for example after sorter.

    attach_mappings = function(prompt_bufnr, map)
      actions.select_default:replace(function()
        actions.close(prompt_bufnr)
        local selection = action_state.get_selected_entry()
        -- print(vim.inspect(selection))
        vim.api.nvim_put({ selection[1] }, "", false, true)
      end)
      return true
    end,

We do this by setting the attach_mappings key to a function. This function needs to return either true or false. If it returns false it means that only the actions defined in the function should be attached. In this case it would remove the default actions to move the selected item in the picker, move_selection_{next,previous}. So in most cases you'll want to return true. If the function does not return anything then an error is thrown.

The attach_mappings function has two parameters, prompt_bufnr is the buffer number of the prompt buffer, which we can use to get the pickers object and map is a function we can use to map actions or functions to arbitrary key sequences.

Now we are replacing select_default the default action, which is mapped to <CR> by default. To do this we need to call actions.select_default:replace and pass in a new function.

In this new function we first close the picker with actions.close and then get the selection with action_state. It's important to notice that you can still get the selection and current prompt input (action_state.get_current_line()) with action_state even after the picker is closed.

You can look at the selection with print(vim.inspect(selection)) and see that it differs from our input (string), this is because internally we pack it into a table with different keys. You can specify this behavior and we'll talk about that in the next section. Now all that is left is to do something with the selection we have. In this case we just put the text in the current buffer with vim.api.nvim_put.

Entry Maker

Entry maker is a function used to transform an item from the finder to an internal entry table, which has a few required keys. It allows us to display one string but match something completely different. It also allows us to set an absolute path when working with files (so the file will always be found) and a relative file path for display and sorting. This means the relative file path doesn't even need to be valid in the context of the current working directory.

We will now try to define our entry maker for our example by providing an entry_maker to finders.new_table and changing our table to be a little bit more interesting. We will end up with the following new code for finders.new_table:

    finder = finders.new_table {
      results = {
        { "red", "#ff0000" },
        { "green", "#00ff00" },
        { "blue", "#0000ff" },
      },
      entry_maker = function(entry)
        return {
          value = entry,
          display = entry[1],
          ordinal = entry[1],
        }
      end
    },

With the new snippet, we no longer have an array of strings but an array of tables. Each table has a color name and the color's hex value.

entry_maker is a function that will receive each table and then we can set the values we need. It's best practice to have a value reference to the original entry, that way we will always have access to the complete table in our action.

The display key is required and is either a string or a function(tbl), where tbl is the table returned by entry_maker. So in this example tbl would give our display function access to value and ordinal.

If our picker will have a lot of values it's suggested to use a function for display, especially if you are modifying the text to display. This way the function will only be executed for the entries being displayed. For an example of an entry maker take a look at lua/telescope/make_entry.lua.

A good way to make your display more like a table is to use a displayer which can be found in lua/telescope/pickers/entry_display.lua. A simpler example of displayer is the function gen_from_git_commits in make_entry.lua.

The ordinal is also required, which is used for sorting. As already mentioned this allows us to have different display and sorting values. This allows display to be more complex with icons and special indicators but ordinal could be a simpler sorting key.

There are other important keys which can be set, but do not make sense in the current context as we are not dealing with files:

path: to set the absolute path of the file to make sure it's always found
lnum: to specify a line number in the file. This will allow the conf.grep_previewer to show that line and the default action to jump to that line.

Previewer

We will not write a previewer for this picker because it isn't required for basic colors and is a more advanced topic. It's already well documented in :help telescope.previewers so you can read this section if you want to write your own previewer. If you want a file previewer without columns you should default to conf.file_previewer or conf.grep_previewer.

Oneshot Job

The oneshot_job finder can be used to have an asynchronous external process which will find results and call entry_maker for each entry. An example usage would be find.

finder = finders.new_oneshot_job({ "find" }, opts ),

More examples

A good way to find more examples is to look into the lua/telescope/builtin directory which contains all of the builtin pickers. Another way to find more examples is to take a look at the extension wiki page as this provides many extensions people have already written which use these concepts.

If you still have any questions after reading this guide please feel free to ask us for more information on gitter and we will happily answer your questions and hopefully allow us to improve this guide. You can also help us to improve this guide by sending a PR.

Bundling as extension

If you now want to bundle your picker as extension, so it is available as picker via the :Telescope command, the following has to be done.

Structure your plugin as follows, so it can be found by telescope:

.
└── lua
    ├── plugin_name             # Your actual plugin code
    │   ├── init.lua
    │   └── some_file.lua
    └── telescope
        └── _extensions         # The underscore is significant
            └─ plugin_name.lua  # Init and register your extension

The lua/telescope/_extensions/plugin_name.lua file needs to return the following: (see :help telescope.register_extension)

return require("telescope").register_extension {
  setup = function(ext_config, config)
    -- access extension config and user config
  end,
  exports = {
    stuff = require("plugin_name").stuff
  },
}

The setup function can be used to access the extension config and setup extension specific global configuration. You also have access to the user telescope default config, so you can override specific internal function. For example sorters if you have an extension that provides a replacement sorter, like telescope-fzf-native.

The exports table declares the exported pickers that can then be accessed via Telescope plugin_name stuff. If you only provide one export it is suggested that you name the key like the plugin, so you can access it with Telescope plugin_name.

Technical

Picker

This section is an overview of how custom pickers can be created and configured.

-- lua/telescope/pickers.lua
Picker:new{
  prompt_title            = "",
  finder                  = FUNCTION, -- see lua/telescope/finders.lua
  sorter                  = FUNCTION, -- see lua/telescope/sorters.lua
  previewer               = FUNCTION, -- see lua/telescope/previewers/previewer.lua
  selection_strategy      = "reset", -- follow, reset, row
  border                  = {},
  borderchars             = {"─", "│", "─", "│", "┌", "┐", "┘", "└"},
  default_selection_index = 1, -- Change the index of the initial selection row
}

Finders

-- lua/telescope/finders.lua
Finder:new{
  entry_maker     = function(line) end,
  fn_command      = function() { command = "", args  = { "ls-files" } } end,
  static          = false,
  maximum_results = false
}

Actions

Overriding actions/action_set

How to override what different functions / keys do.

TODO: Talk about what actions vs actions sets are

Relevant Files

lua/telescope/actions/init.lua
- The most "user-facing" of the files, which has the builtin actions that we provide
lua/telescope/actions/set.lua
- The second most "user-facing" of the files. This provides actions that are consumed by several builtin actions, which allows for only overriding ONE item, instead of copying the same configuration / function several times.
lua/telescope/actions/state.lua
- Provides APIs for interacting with the state of telescope from within actions.
- These are useful for writing your own actions and interacting with telescope
lua/telescope/actions/mt.lua
- You probably don't need to look at this, but it defines the behavior of actions.

`:replace(function)`

Directly override an action with a new function

local actions = require('telescope.actions')
actions.select_default:replace(git_checkout_function)

`:replace_if(conditional, function)`

Override an action only when conditional returns true.

local action_set = require('telescope.actions.set')
action_set.select:replace_if(
  function()
    return action_state.get_selected_entry().path:sub(-1) == os_sep
  end, function(_, type)
    -- type is { "default", "horizontal", "vertical", "tab" }
    local path = actions.get_selected_entry().path
    action_state.get_current_picker(prompt_bufnr):refresh(gen_new_finder(new_cwd), { reset_prompt = true})
  end
)

`:replace_map(configuration)`

local action_set = require('telescope.actions.set')
-- Use functions as keys to map to which function to execute when called.
action_set.select:replace_map {
  [function(e) return e > 0 end] = function(e) return (e / 10) end,
  [function(e) return e == 0 end] = function(e) return (e + 10) end,
}

Previewers

See :help telescope.previewers

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