BNFC.Backend.Base

Backend base module.

Defines the type of the backend and some useful functions.

Documentation

type Backend = MkFiles ()

type MkFiles a = WriterT [GeneratedFile] IO a

Define the type of the backend functions. For more purity, instead of having each backend writing the generated files to disk, they return a list of pairs containing the (relative) file path and the file content. This allow for 1) easier testing, 2) implement common options like changing the output dir or providing a diff instead of overwritting the files on a highter level and 3) more purity.

The writer monad provides a more convenient API to generate the list. Note that we still use the IO monad for now because some backends insist on printing stuff to the screen while generating the files.

data GeneratedFile

A result file of a backend.

Constructors

GeneratedFile

Fields

• fileName :: FilePath

  Name of the file to write.

• makeComment :: MakeComment

  Function to generate a comment. Used to prefix the file with a stamp ("Generated by BNFC").

• fileContent :: String

  Content of the file to write.

Instances

Show GeneratedFile

Defined in BNFC.Backend.Base

Methods

showsPrec :: Int -> GeneratedFile -> ShowS

show :: GeneratedFile -> String

showList :: [GeneratedFile] -> ShowS

Eq GeneratedFile

Defined in BNFC.Backend.Base

Methods

(==) :: GeneratedFile -> GeneratedFile -> Bool

(/=) :: GeneratedFile -> GeneratedFile -> Bool

type MakeComment = String -> String

Type of comment-generating functions.

execBackend :: MkFiles () -> IO [GeneratedFile]

Named after execWriter, this function execute the given backend and returns the generated file paths and contents.

mkfile :: FileContent c => FilePath -> MakeComment -> c -> MkFiles ()

A specialized version of tell that adds a file and its content to the list of generated files.

liftIO :: MonadIO m => IO a -> m a

Lift a computation from the IO monad. This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations (i.e. IO is the base monad for the stack).

Example

import Control.Monad.Trans.State -- from the "transformers" library

printState :: Show s => StateT s IO ()
printState = do
  state <- get
  liftIO $ print state

Had we omitted liftIO, we would have ended up with this error:

• Couldn't match type ‘IO’ with ‘StateT s IO’
 Expected type: StateT s IO ()
   Actual type: IO ()

The important part here is the mismatch between StateT s IO () and IO ().

Luckily, we know of a function that takes an IO a and returns an (m a): liftIO, enabling us to run the program and see the expected results:

> evalStateT printState "hello"
"hello"

> evalStateT printState 3
3

writeFiles :: FilePath -> MkFiles () -> IO ()

Write a set of files to disk. the first argument is the root directory inside which all the generated files will be written. This root directory and sub-directories will be created as needed (ex: if the files contains a a/b/file.txt, writeFiles will create the directories $ROOT/a and $ROOT/a/b)