View patterns for validation

GHC's ViewPatterns extension has a lot of unexpected uses. One that I've found recently is input validation.

{-# LANGUAGE ViewPatterns #-}

import Control.Monad
import Text.Printf

months :: [String]
months = words "Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec"

Using this function:

range :: (Ord a) => a -> a -> a -> a
range lb ub x
  | (x < lb) || (x > ub) = error "argument out of range"
  | otherwise = x

we can put bounds on arguments:

month :: Int -> String
month (range 1 12 -> m) = months !! (m-1)

like so:

GHCi> month 3
"Mar"
GHCi> month 15
"*** Exception: argument out of range

This version provides better error messages:

-- V for 'verbose'
rangeV :: (Ord a, Show a) => String -> a -> a -> a -> a
rangeV fun lb ub x
  | (x < lb) || (x > ub) = error (printf msg fun (show x) (show lb) (show ub))
  | otherwise = x
  where msg = "%s: argument %s is outside of range [%s,%s]"

like so:

monthV :: Int -> String
monthV (rangeV "monthV" 1 12 -> m) = months !! (m-1)

GHCi> monthV 3
"Mar"
GHCi> monthV 15
"*** Exception: monthV: argument 15 is outside of range [1,12]

Or handle failure monadically:

-- Mp for MonadPlus
rangeMp :: (MonadPlus m, Ord a) => a -> a -> a -> m a
rangeMp lb ub x = guard ((x >= lb) && (x <= ub)) >> return x

like so:

monthMaybe :: Int -> Maybe String
monthMaybe (rangeMp 1 12 -> Just m) = Just (months !! (m-1))
monthMaybe _ = Nothing

GHCi> monthMaybe 3
Just "Mar"
GHCi> monthMaybe 15
Nothing