Recover in Idris

In Idris, we can use the IO monad to handle effects like printing to the console. Here’s how we can implement a program that demonstrates error recovery:

import System.IO

-- This function throws an exception
mayThrow : IO ()
mayThrow = putStrLn "Throwing an exception" >> fail "a problem"

-- Main function that demonstrates error recovery
main : IO ()
main = do
  putStrLn "Starting the program"
  result <- try mayThrow
  case result of
    Left err -> putStrLn $ "Recovered. Error:\n" ++ show err
    Right _ -> putStrLn "No error occurred"
  putStrLn "After mayThrow()"

In Idris, we use the try function to catch exceptions. The try function returns a value of type Either a b, where Left a represents an error and Right b represents a successful result.

The mayThrow function simulates a function that might throw an exception. In this case, it always throws an exception using the fail function.

In the main function, we use try to call mayThrow. We then use pattern matching to handle the result:

• If an error occurred (represented by Left err), we print the error message.
• If no error occurred (represented by Right _), we print a message indicating that no error occurred.

After the try block, we print a message to show that the program continues executing.

To run this program, save it in a file with a .idr extension (e.g., ErrorRecovery.idr), then use the Idris compiler to compile and run it:

$ idris ErrorRecovery.idr -o ErrorRecovery
$ ./ErrorRecovery
Starting the program
Throwing an exception
Recovered. Error:
User error: a problem
After mayThrow()

This example demonstrates how Idris handles error recovery, which is conceptually similar to recovering from panics in other languages. The try function in Idris allows us to catch and handle exceptions, preventing them from crashing the program and allowing for graceful error handling.