Defer in Idris

In Idris, we don’t have a direct equivalent of the defer keyword. However, we can achieve similar functionality using the finally function from the Control.Exception module. This function ensures that a cleanup action is performed regardless of whether an exception is thrown or not.

Let’s see how we can implement the file creation, writing, and closing example in Idris:

import System.File
import System.FFI
import Control.Exception

-- Suppose we wanted to create a file, write to it,
-- and then close when we're done. Here's how we could
-- do that with `finally`.

main : IO ()
main = do
  -- We use `finally` to ensure that the file is closed
  -- after we're done writing to it, even if an exception occurs.
  Right () <- finally (do
    Right f <- openFile "/tmp/defer.txt" WriteTruncate
      | Left err => putStrLn $ "Error opening file: " ++ show err
    writeFile f "data"
    ) (closeFile f)
    | Left err => putStrLn $ "Error: " ++ show err
  pure ()

-- Helper functions

createFile : String -> IO (Either FileError File)
createFile p = do
  putStrLn "creating"
  openFile p WriteTruncate

writeFile : File -> String -> IO ()
writeFile f content = do
  putStrLn "writing"
  fPutStrLn f content

closeFile : File -> IO ()
closeFile f = do
  putStrLn "closing"
  fclose f

In this Idris version:

1. We use finally from Control.Exception to ensure that the file is closed after we’re done with it, even if an exception occurs.

2. The createFile function opens a file for writing, similar to the Go version.

3. The writeFile function writes a string to the file.

4. The closeFile function closes the file.

5. In the main function, we use pattern matching with Right and Left to handle potential errors, as Idris uses Either for error handling.

6. We’ve simplified the error handling compared to the Go version. In a real application, you might want to add more detailed error handling.

Running this program would produce output similar to the Go version:

$ idris defer.idr -o defer
$ ./defer
creating
writing
closing

Note that Idris’s type system and its approach to IO and error handling are quite different from Go’s. This example demonstrates how to achieve similar functionality, but it’s important to remember that idiomatic Idris code might approach this problem differently, possibly using more advanced features of the language’s type system.