Maintainer

CS 131, Programming Languages (Melissa O'Neill, Chris Stone, Ben Wiedermann) This module provides a simple monad for handling computations that can fail with an error. For example: Fallible a is either - a thing (of type a) or - a failure with an error message Unlike Haskell's error function, for computation to continue, Haskell must *know* whether the previous computation failed or not. Thus let err = error "Oops" in 42 will produce 42 without ever showing an error, in contrast do err <- fail "Oops" return 42 will fail and never even try to compute 42. On the negative side, it does impose *some* sequencing, in that let a = foo "Wow" b = bar 42 in [b,a] will produce a list, then compute b if it is needed, then a if it is needed. The *same* would occur if we wrote: do a <- return (foo "Wow") b <- return (bar 42) return [b,a] but if foo and bar return Fallible, if we write do a <- foo "Wow" b <- bar 42 return [b,a] Haskell must first evaluate foo *enough* to know whether or not it failed, and then, if it succeeded, evaluate bar enough to know whether it failed, and then if that succeeded, return a and b. a and b themselves may not yet have been computed (due to laziness) but we at least know that we have a "successful" result.

Safe Haskell

Safe

Fallible

Description

Synopsis

Documentation

data Fallible a #

A Fallible can succeed with a result or fail with a message

Constructors

Result a
Failure String

Instances

Monad Fallible #
Methods (>>=) :: Fallible a -> (a -> Fallible b) -> Fallible b (>>) :: Fallible a -> Fallible b -> Fallible b return :: a -> Fallible a fail :: String -> Fallible a
Functor Fallible #
Methods fmap :: (a -> b) -> Fallible a -> Fallible b (<$) :: a -> Fallible b -> Fallible a
MonadFail Fallible #
Methods fail :: String -> Fallible a
Applicative Fallible #
Methods pure :: a -> Fallible a (<>) :: Fallible (a -> b) -> Fallible a -> Fallible b (>) :: Fallible a -> Fallible b -> Fallible b (<*) :: Fallible a -> Fallible b -> Fallible a
MonadPlus Fallible #
Methods mzero :: Fallible a mplus :: Fallible a -> Fallible a -> Fallible a
Alternative Fallible #
Methods empty :: Fallible a (<\|>) :: Fallible a -> Fallible a -> Fallible a # some :: Fallible a -> Fallible [a] # many :: Fallible a -> Fallible [a] #
Eq a => Eq (Fallible a) #
Methods (==) :: Fallible a -> Fallible a -> Bool (/=) :: Fallible a -> Fallible a -> Bool
Ord a => Ord (Fallible a) #
Methods compare :: Fallible a -> Fallible a -> Ordering (<) :: Fallible a -> Fallible a -> Bool (<=) :: Fallible a -> Fallible a -> Bool (>) :: Fallible a -> Fallible a -> Bool (>=) :: Fallible a -> Fallible a -> Bool max :: Fallible a -> Fallible a -> Fallible a min :: Fallible a -> Fallible a -> Fallible a
Show a => Show (Fallible a) #
Methods showsPrec :: Int -> Fallible a -> ShowS show :: Fallible a -> String showList :: [Fallible a] -> ShowS

fail2error :: Fallible a -> a #

Pull the result out of the Fallible or display an error message.

catchfail :: (String -> a) -> Fallible a -> a #

Handle a failure (or produce the result if the computation succeeded).