Shellout.hs

{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ViewPatterns #-}
{-|
Module      : Foreign.Nix.Shellout
Description : Interface to the nix package manager’s CLI
Copyright   : Profpatsch, 2016–2018
License     : GPL-3
Stability   : experimental
Portability : nix 1.11.x, nix 2.0

Calls to the nix command line to convert
textual nix expressions to derivations & realized storepaths.
-}
module Foreign.Nix.Shellout
( -- * Calling nix
  -- ** Parse
  parseNixExpr, ParseError(..)
  -- ** Instantiate
, instantiate, InstantiateError(..)
, eval
  -- ** Realize
, realize, RealizeError(..)
  -- ** Helpers
, addToStore
, parseInstRealize
, NixError(..)
  -- * Types
, StorePath(..), Derivation, Realized
, NixExpr
, runNixAction, NixAction(..), NixActionError(..)
) where

import Control.Error ( throwE, tryLast )
import Data.Text (stripPrefix, lines, isPrefixOf, Text)

import qualified Foreign.Nix.Shellout.Helpers as Helpers
import Foreign.Nix.Shellout.Types
    ( Realized,
      Derivation,
      StorePath(..),
      NixActionError(..),
      NixAction(..),
      runNixAction )
import qualified Data.Text as Text
import System.Exit (ExitCode(ExitFailure, ExitSuccess))
import Data.Bifunctor (bimap, Bifunctor (first))
import Control.Monad ((>=>))
import qualified System.FilePath as FilePath
import Data.Function ((&))
import qualified Data.List as List

------------------------------------------------------------------------------
-- Parsing

-- | A sucessfully parsed nix expression.
newtype NixExpr = NixExpr Text deriving (Show)

data ParseError
  = SyntaxError Text
    -- ^ the input string was not a syntactically valid nix expression
  | UnknownParseError
    -- ^ catch-all error
  deriving (Show, Eq)

-- | Parse a nix expression and check for syntactic validity.
parseNixExpr :: Text -> NixAction ParseError NixExpr
parseNixExpr e =
  bimap parseParseError NixExpr
    $ evalNixOutput "nix-instantiate" [ "--parse", "-E", e ]


parseParseError :: Text -> ParseError
parseParseError
  (stripPrefix "error: syntax error, "
               -> Just mes) = SyntaxError $ mes
parseParseError _           = UnknownParseError

------------------------------------------------------------------------------
-- Instantiating

data InstantiateError
  = NotADerivation
    -- ^ the given expression does not evaluate to a derivaton
  | UnknownInstantiateError
    -- ^ catch-all error
  deriving (Show, Eq)

-- | Instantiate a parsed expression into a derivation.
instantiate :: NixExpr -> NixAction InstantiateError (StorePath Derivation)
instantiate (NixExpr e) =
  first parseInstantiateError
    $ evalNixOutput "nix-instantiate" [ "-E", e ]
      >>= toNixFilePath StorePath

-- | Just tests if the expression can be evaluated.
-- That doesn’t mean it has to instantiate however.
eval :: NixExpr -> NixAction InstantiateError ()
eval (NixExpr e) = do
  _instantiateOutput <- first parseInstantiateError
       $ evalNixOutput "nix-instantiate" [ "--eval", "-E", e ]
  pure ()

parseInstantiateError :: Text -> InstantiateError
parseInstantiateError
  (stripPrefix "error: expression does not evaluate to a derivation"
               -> Just _) = NotADerivation
parseInstantiateError _   = UnknownInstantiateError


------------------------------------------------------------------------------
-- Realizing

data RealizeError = UnknownRealizeError deriving (Show, Eq)

-- | Finally derivations are realized into full store outputs.
-- This will typically take a while so it should be executed asynchronously.
realize :: StorePath Derivation -> NixAction RealizeError (StorePath Realized)
realize (StorePath d) =
     storeOp [ "-r", Text.pack d ]

-- | Copy the given file or folder to the nix store and return it’s path.
addToStore :: FilePath -> NixAction RealizeError (StorePath Realized)
addToStore fp = storeOp [ "--add", Text.pack fp ]

storeOp :: [Text] -> NixAction RealizeError (StorePath Realized)
storeOp op =
  first (const UnknownRealizeError)
    $ evalNixOutput "nix-store" op
      >>= toNixFilePath StorePath

------------------------------------------------------------------------------
-- Convenience

-- | Combines all error types that could happen.
data NixError
  = ParseError ParseError
  | InstantiateError InstantiateError
  | RealizeError RealizeError deriving (Show, Eq)

-- | A convenience function to directly realize a nix expression.
-- Any errors are put into a combined error type.
parseInstRealize :: Text -> NixAction NixError (StorePath Realized)
parseInstRealize = first ParseError . parseNixExpr
               >=> first InstantiateError . instantiate
               >=> first RealizeError . realize

------------------------------------------------------------------------------
-- Helpers

-- | Take args and return either error message or output path
evalNixOutput :: Text
              -- ^ name of executable
              -> [Text]
              -- ^ arguments
              -> NixAction Text Text
              -- ^ error: (stderr, errormsg), success: path
evalNixOutput = Helpers.readProcess (\(out, err) -> \case
  ExitFailure _ -> throwE $
    case
      err
        & Text.lines
        & dropWhile (not . isPrefixOf "error: ")
        & List.intersperse "\n"
        & mconcat of
      "" -> "nix didn’t output any error message"
      s  -> s
  ExitSuccess -> tryLast
      "nix didn’t output a store path" (Data.Text.lines out))


-- | Apply filePath p to constructor a if it’s a valid filepath
toNixFilePath :: (String -> a) -> Text -> NixAction Text a
toNixFilePath a p = NixAction $
  if FilePath.isValid (Text.unpack p) then pure $ a (Text.unpack p)
  else throwE $ NixActionError
          { actionStderr = nostderr
          , actionError = p <> " is not a filepath!" }
  where nostderr = mempty