Complete Monad Example.md

Example:

In the context of a job application, we have a type class Candidate

data Candidate = Candidate
   { candidateId :: Int
   , codeReview :: Grade
   , cultureFit :: Grade
   , education :: Degree } deriving Show

Where Grade and Degree are defined as,

data Grade = F | D | C | B | A deriving (Eq, Ord, Enum, Show, Read)
data Degree = HS | BA | MS | PhD deriving (Eq, Ord, Enum, Show, Read)

We can create a random candidate like so,

adityaVerma = Candidate { candidateId = 1, codeReview = A, cultureFit = B, education = BA }

To check if the candidate is good we define the function,

viable :: Candidate -> Bool
viable candidate = all (== True) tests
   where passedCoding = codeReview candidate > B
         passedCultureFit = cultureFit candidate > C
         educationMin = education candidate >= MS
         tests = [passedCoding
                 ,passedCultureFit
                 ,educationMin]   

This method above takes a candidate and returns a bool.

What if we need to operate on a candidate that is in an IO Context, or Maybe (after having been retrieved from a DB), or a whole list of candidates.

A truly generic function to access candidate, one which can take any monad

assess :: (Monad m) => m Candidate -> m String 
assess candidateInContext = do
                            candidate <- candidateInContext
                           	let isPassed = if (viable candidate) then "passed"
                                           else "fail"
                            return isPassed

• This method takes in a candidateInContext
• After do, using <-, we extract the individual element from the context
• In the let we operate on the individual element, transforming it into a "passed" or a "failed"
• return helps us wrap "Passed" or "failed" back to the context

To test this method out, in Maybe context

> assess (Just adityaVerma )
Just "fail"

In a list

> assessCandidateX [adityaVerma , candidate1, candidate2]
["fail","fail","pass"]

In IO, we need a helper function to read the element from IO

readCandidate :: IO Candidate
readCandidate = do
   putStrLn "enter id:"
   cId <- readInt
   putStrLn "enter code grade:"
   codeGrade <- readGrade
   putStrLn "enter culture fit grade:"
   cultureGrade <- readGrade
   putStrLn "enter education:"
   degree <- readDegree
   return (Candidate { candidateId = cId
                     , codeReview = codeGrade
                     , cultureFit = cultureGrade
                     , education = degree })

The method above takes values from the CLI and returns an IO Candidate. To test the method out.

> assess readCandidate
enter id:
1
enter code grade:
A
enter culture fit grade:
A
enter education:
MS
"passed"