Note: this is the stubbed version of module MaybePractice. Try to figure out how to fill in all parts of this file marked undefined. CIS 552 students should be able to access this code through github. Eventually, the completed version will be available.

In class exercise: Practice with Maybe Monad

The goal of this short in-class exercise is to get a bit more practice with using the Maybe Monad.

> module MaybePractice where

> import qualified Control.Monad as Monad

> import Data.Map (Map)
> import qualified Data.Map as Map
> import qualified Text.Read as Text

Part 1

Consider the Weather datatype from the Kata problem from HW #3.

> data Weather = Weather {
>     dayNumber :: Int, maxTemp :: Int, minTemp :: Int 
>     } deriving (Eq, Show)

Use Map.lookup and Text.readMaybe to write a function that takes a dictionary (i.e. a finite map from String keys to String values) and converts it into weather data. Your implementation should not use pattern matching; use do notation instead.

If the provided dictionary does not have the appropriate data, then the function should return Nothing. In other words, this function should behave according to the examples below.

> -- >>> parseWeather (Map.fromList [("day", "2"), ("maxTemp", "78"), ("minTemp", "62")])
> -- Just (Weather {dayNumber = 2, maxTemp = 78, minTemp = 62})
> -- >>> parseWeather (Map.fromList [("day", "2")])
> -- Nothing
> -- >>> parseWeather (Map.fromList [("day", "two"), ("maxTemp", "78"), ("minTemp", "62")])
> -- Nothing

> parseWeather :: Map String String -> Maybe Weather
> parseWeather = undefined

Part 2

Consider the following operations that combine two Maybes. Implement them both. One can be defined using (>>=) from the Maybe monad, and one cannot. Which is which?

> -- | Left-biased choice on maybes
> --
> -- >>> firstJust (Just 1) (Just 2)
> -- Just 1
> -- >>> firstJust Nothing (Just 2)
> -- Just 2
> -- >>> firstJust (Just 1) Nothing
> -- Just 1
> -- >>> firstJust Nothing Nothing
> -- Nothing
> firstJust :: Maybe a -> Maybe a -> Maybe a
> firstJust = undefined

> -- | Ensure that both Maybes are 'Just' and retain the first one
> -- 
> -- >>> sequenceFirst (Just 1) (Just 'a')
> -- Just 1
> -- >>> sequenceFirst Nothing (Just 'a')
> -- Nothing
> -- >>> sequenceFirst (Just 1) Nothing
> -- Nothing
> -- >>> sequenceFirst Nothing Nothing
> -- Nothing

> sequenceFirst :: Maybe a -> Maybe b -> Maybe a
> sequenceFirst = undefined

CIS 552: Advanced Programming

Fall 2021

In class exercise: Practice with Maybe Monad

Part 1

Part 2