How to chain the use of maybe argument in haskell?

Question

I\'m trying to build a string from optional arguments. For example to generate a greeting string from a title and a name This is trivial in a imperative language and would l

Answer 1

I think that your function is violating the SRP (Single responsibility principle). It is doing two things:

• prefixing the name with Mr
• rendering the greeting message

You can notice that the Bool and String (name) refer to the same "entity" (a person).

So let's define a person:

data Person 
    = Mister String
    | Person String
    deriving (Eq)

We can now create an instance of show that makes sense:

instance Show Person where
    show (Mister name) = "Mr. " ++ name
    show (Person name) = name

and finally we can reformulate your greeting function as:

greeting :: Maybe Person -> String
greeting (Just person) = "Hello, " ++ show person
greeting Nothing       = "Hello"

Live demo

The code is simple, readable and just few lines longer (don't be afraid of writing code). The two actions (prefixing and greeting) are separated and everything is much simpler.

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If you really have to, you can trivially create a function to generate a Mister or a Person based on a boolean value:

makePerson :: Bool -> String -> Person
makePerson True  = Mister
makePerson False = Person

Live demo