I have created a data group:
data Cars = vw | audi | ford
data Engine = TDI | TSI
I want to use these in a function/list comprehension. To show combinations I have this:
[(x,y) | x <- Cars , y <- Engine]
But I'mm a little bit lost on how to use the data within a function.
I get this error:
Not in scope: data constructor `Cars'
Cars and Engine are types, nothing more, especially not lists of constructors. To get a collection of all constructors of a type, make it an instance of Enum and perhaps Bounded (you can derive both, Bounded is not necessary but nice as it allows not hardcoding the start value). See e.g. this question (in particular, this answer). Copying and adjusting that code (also, I fixed the casing of the ctors and renamed Cars as it was plural and not very specific):
data CarManufacturer = VW | Audi | Ford deriving (Bounded, Enum)
data Engine = TDI | TSI deriving (Bounded, Enum)
allValues :: (Bounded a, Enum a) => [a]
allValues = [minBound..]
allEngines = allValues :: [Engine]
allCars = allValues :: [CarManufacturer]
carsWithEngines = [(car, engine) | car <- allCars, engine <- allEngines]
deriving clauses to your code.
allValues function, that never occurred to me. I know plenty of cases where this could save some code in my programs.Is that what you want?
$cat t.hs
data Cars = Vw | Audi | Ford
deriving (Enum, Show)
data Engine = TDI | TSI
deriving (Enum, Show)
main = print [(x,y) | x <- [Vw .. Ford] , y <- [TDI .. TSI]]
$ghc t.hs
$./a.out
[(Vw,TDI),(Vw,TSI),(Audi,TDI),(Audi,TSI),(Ford,TDI),(Ford,TSI)]
Unfortunately you can't access the constructors of a datatype this way. First, all data constructors need to be capitalized, so change to this:
data Cars = VW | Audi | Ford deriving Show
data Engine = TDI | TSI deriving Show
-- create explicit lists of the constructors
carTypes :: [Cars]
carTypes = [VW, Audi, Ford]
engineTypes :: [Engine]
engineTypes = [TDI, TSI]
now you can use these lists in a list comprehension
> let ll = [(x,y) | x <- carTypes, y <- engineTypes]
> show ll
> [(VW,TDI),(VW,TSI), (Audi,TDI),(Audi,TSI),(Ford,TDI),(Ford,TSI)]
> :type ll
> ll :: [(Cars,Engine)]
