Add a function to run MonadNixThunk to make the library easier to use

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+* [#45] Add runMonadNixThunk to make it easier to run the actions defined in the library.
 * [#42](https://github.com/obsidiansystems/nix-thunk/pull/42) Thunk read errors are now presented in a more informative manner.
 * [#43](https://github.com/obsidiansystems/nix-thunk/pull/43) `nix-thunk` will now ensure that any `git` processes invoked during its execution have a clean configuration. 
   This prevents `nix-thunk` crashing when e.g. the user's configuration `git` is valid only in a version newer than what `nix-thunk` links against, and works towards making thunks more reproducible by ensuring that thunk URIs are resolvable independently of the user's environment.

nix-thunk.cabal

@@ -1,6 +1,6 @@
 cabal-version:      >=1.10
 name:               nix-thunk
-version:            0.6.1.0
+version:            0.6.2.0
 license:            BSD3
 license-file:       LICENSE
 copyright:          Obsidian Systems LLC 2020-2022

src/Nix/Thunk.hs

@@ -9,11 +9,13 @@
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE PatternGuards #-}
 {-# LANGUAGE QuasiQuotes #-}
+{-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeApplications #-}
 module Nix.Thunk
-  ( ThunkSource (..)
+  ( runMonadNixThunk
+  , ThunkSource (..)
   , GitHubSource (..)
   , ThunkRev (..)
   , getLatestRev

src/Nix/Thunk/Internal.hs

@@ -8,6 +8,7 @@
 {-# LANGUAGE MultiWayIf #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE PatternGuards #-}
+{-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE QuasiQuotes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TemplateHaskell #-}
@@ -73,6 +74,7 @@ import Data.Yaml (parseMaybe)
 import GitHub
 import GitHub.Data.Name
 import System.Directory
+import System.Environment (getArgs)
 import System.Exit
 import System.FilePath
 import System.IO.Error (isDoesNotExistError)
@@ -97,6 +99,12 @@ type MonadNixThunk m =
   , MonadFail m
   )
 
+-- | Runs MonadNixThunk actions with relatively sensible options for non-interactive use.
+runMonadNixThunk :: forall a. (forall m. MonadNixThunk m => m a) -> IO a
+runMonadNixThunk x = do
+  cliConf <- newCliConfig Error False False (\e -> (prettyNixThunkError e, ExitFailure 1))
+  runCli cliConf x
+
 data NixThunkError
    = NixThunkError_ProcessFailure ProcessFailure
    | NixThunkError_Unstructured Text