CODEWITHSUNDEEP
functionhaskelltypestypeclass
FastBatteryCharger
FastBatteryCharger

Reputation: 349

Haskell, data type function and how to extract information?

i created two data types which let me create a "Hill" and a "MountainChain":

data Hill = Hill Name Size Isolation Prominence
  deriving(Eq,Show)
type Name      = String
type Size     = Float
type Isolation  = Float
type Prominence = Float

data MountainChain = MountainChain Name [Hill]
  deriving(Eq,Show)

Example:

fstHill = Hill "fstHill" 45.0 18.0 4.0

scdHill = Hill "scdHill" 222.0 60.0 15.0

output: 

*Main> fstHill
Hill "fstHill" 45.0 18.0 4.0
*Main> scdHill
Hill "scdHill" 222.0 60.0 15.0

My question now is how to write a function that will return the maxSize of an Hill within the MountainChain and a function that returns the name of the highest Hill (within the MC) with the following types:

mSize :: MountainChain -> Size
majorHill :: MountainChain -> Name

Upvotes: 0

Views: 679

Answers (1)

Jules
Jules

Reputation: 14520

You can implement mSize easily using pattern matching and a helper function:

maxSize' :: Hill -> (Size, Name)
maxSize' (Hill n s _ _) = (s, n)

maxSize :: MountainChain -> Size
maxSize (MountainChain _ mc) = fst $ maximum $ maxSize' <$> mc

majorHill :: MountainChain -> Name
majorHill (MountainChain _ mc) = snd $ maximum $ maxSize' <$> mc

What's happening here is that maxSize does a pattern match against the Hill type and extracts the name and size. It then takes the value of a tuple with the size as its first element (there's a reason for that, and I'll get to it in a bit).

maxSize just maps maxSize' over a MountainChain, which (thanks to pattern matching) is treated just like a list of Hills. We then apply maximum over the returned list of [(Float, String)]. maximum is fortunately smart enough that it'll perform a stable sort on a list of tuples using the first element in each tuple. Finally, we extract the first element from the single tuple returned by maximum using fst.

majorHill once again makes use of this mechanic, but extracts the name (using snd) instead of the height.

Upvotes: 1

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