feat: setup configuration with default fallbacks

Summary:

Now the users configuration and the default configuration is separated. This
will make it easier to setup ivy with some defaults. Right now we only have the
backends configurable however, it looks like we will be making other components
of ivy configurable.

To use this you can now call `ivy.setup` with no parameters and you will get
the default config. You can also call it with a partial config and if the
option is not found in the users config it will fallback to the users config
value.

Test Plan:

Manual testing and with unit tests in CI

lua/ivy/config.lua

@@ -0,0 +1,33 @@
+local config_mt = {}
+config_mt.__index = config_mt
+
+function config_mt:get_in(config, key_table)
+  local current_value = config
+  for _, key in ipairs(key_table) do
+    if current_value == nil then
+      return nil
+    end
+
+    current_value = current_value[key]
+  end
+
+  return current_value
+end
+
+function config_mt:get(key_table)
+  return self:get_in(self.user_config, key_table) or self:get_in(self.default_config, key_table)
+end
+
+local config = { user_config = {} }
+
+config.default_config = {
+  backends = {
+    "ivy.backends.buffers",
+    "ivy.backends.files",
+    "ivy.backends.lines",
+    "ivy.backends.rg",
+    "ivy.backends.lsp-workspace-symbols",
+  },
+}
+
+return setmetatable(config, config_mt)

lua/ivy/config_spec.lua

@@ -0,0 +1,27 @@
+local config = require "ivy.config"
+
+describe("config", function()
+  before_each(function()
+    config.user_config = {}
+  end)
+
+  it("gets the first item when there is only default values", function()
+    local first_backend = config:get { "backends", 1 }
+    assert.is_equal("ivy.backends.buffers", first_backend)
+  end)
+
+  it("returns nil if we access a key that is not a valid config item", function()
+    assert.is_nil(config:get { "not", "a", "thing" })
+  end)
+
+  it("returns the users overridden config value", function()
+    config.user_config = { backends = { "ivy.my.backend" } }
+    local first_backend = config:get { "backends", 1 }
+    assert.is_equal("ivy.my.backend", first_backend)
+  end)
+
+  it("returns a nested value", function()
+    config.user_config = { some = { nested = "value" } }
+    assert.is_equal(config:get { "some", "nested" }, "value")
+  end)
+end)

lua/ivy/init.lua

@@ -1,28 +1,32 @@
 local controller = require "ivy.controller"
+local config = require "ivy.config"
 local register_backend = require "ivy.register_backend"
 
--- Local variable to check if ivy has been setup, this is to prevent multiple
--- setups of ivy
-local has_setup = false
-
 local ivy = {}
 ivy.run = controller.run
 ivy.register_backend = register_backend
 
+-- Private variable to check if ivy has been setup, this is to prevent multiple
+-- setups of ivy. This is only exposed for testing purposes.
+---@private
+ivy.has_setup = false
+
 ---@class IvySetupOptions
 ---@field backends (IvyBackend | { ["1"]: string, ["2"]: IvyBackendOptions} | string)[]
 
----@param config IvySetupOptions
-function ivy.setup(config)
-  if has_setup then
+---@param user_config IvySetupOptions
+function ivy.setup(user_config)
+  if ivy.has_setup then
     return
   end
 
-  for _, backend in ipairs(config.backends) do
+  config.user_config = user_config or {}
+
+  for _, backend in ipairs(config:get { "backends" } or {}) do
     register_backend(backend)
   end
 
-  has_setup = true
+  ivy.has_setup = true
 end
 
 return ivy

lua/ivy/init_spec.lua

@@ -0,0 +1,30 @@
+local ivy = require "ivy"
+local config = require "ivy.config"
+
+describe("ivy.setup", function()
+  before_each(function()
+    ivy.has_setup = false
+    config.user_config = {}
+  end)
+
+  it("sets the users config options", function()
+    ivy.setup { backends = { "ivy.backends.files" } }
+    assert.is_equal("ivy.backends.files", config:get { "backends", 1 })
+  end)
+
+  it("will not reconfigure if its called twice", function()
+    ivy.setup { backends = { "ivy.backends.files" } }
+    ivy.setup { backends = { "some.backend" } }
+    assert.is_equal("ivy.backends.files", config:get { "backends", 1 })
+  end)
+
+  it("does not crash if you don't pass in any params to the setup function", function()
+    ivy.setup()
+    assert.is_equal("ivy.backends.buffers", config:get { "backends", 1 })
+  end)
+
+  it("will fallback if the key is not set at all in the users config", function()
+    ivy.setup { some_key = "some_value" }
+    assert.is_equal("ivy.backends.buffers", config:get { "backends", 1 })
+  end)
+end)