I'm in a situation where I use a tuple defined as type synonym, so I can't really use record structure. Is there a way to get these elements?
To be more specific here is what I mean:
type Movie = (Title, Regisseur, MainActors, ReleaseDate, Genre, SalesPrice)
type Title = String
type SalesPrice = Int
etc
How can I get only Title and ReleaseDate for an example. Besides than defining a getter function for every position in the tuple of course.
One other option is to import the package Control.Lens.
>>> import Control.Lens
>>> let movie = ( "The Terminator"
, "James Cameron"
, ["Arnold Schwartzenegger", "Michael Biehn"]
, 1984
,"Science Fiction"
, 19.99)
>>> movie ^. _1
"The Terminator"
>>> movie ^. _4
1984
I wouldn't recommend that you do this though. Use record syntax instead.
data Movie = Movie
{ title :: String
, director :: String
, mainActors :: [String]
, releaseDate :: Int
, genre :: String
, price :: Double
} deriving (Eq,Ord,Show)
Now you can do
>>> let movie = Movie "The Terminator" "James Cameron"
["Arnold Schwartzenegger", " Michael Biehn"]
1984 "Science Fiction" 19.99
>>> title movie
"The Terminator"
>>> releaseDate movie
1984
You need to define (or derive) a getter for every component of the tuple.
E.g.
title :: Movie -> Title
title (t,_,_,_,_,_,_) = t
Such accessors can be made simpler by using records instead.
You can use uniplate to get all tuple elements of a specific type using uniplate. Note that uniplate will treat different type synonyms as if they are the same.
import Data.Generics.Uniplate.Data
.
.
.
let [title] = childrenBi movie
If you want something that differentiates between fields, it might be better to use record syntax: Haskell record syntax
