I am ploughing through Learn You a Haskell for Great Good, and I have reached up to section 8.4, "Derived Instances". In this section, there's the following data type declaration:
data Person = Person { firstName :: String
, lastName :: String
, age :: Int
} deriving (Eq)
While trying
*Main> mikeD == Person {firstName = "Michael", lastname = "Diamond", age = 43}
I got the following error:
<interactive>:55:41:
`lastname' is not a (visible) field of constructor `Person'
By correcting lastname
to lastName
I removed the error.
Question:
In the error message the word (visible)
hints to me that there must be the possibility of declaring a field as hidden / invisible. Is this correct or not? If it is, how can I declare a field in the constructor as hidden, and what are the general scenarios where one would need to declare hidden fields? If you could explain this by giving a simple example of their use, that would be appreciated.
Note: I could not find any reference to/details about hidden or invisible field in LYAH.
It is possible to hide a field of a record, or a constructor of any data type, although not at the declaration site. The idea is to simply choose not to export that constructor and/or field from the module, like so:
Note that everything inside
MyModule
still has access to both constructors and all four fields, but in some other module that importsMyModule
only the exported ones are visible.