CODEWITHSUNDEEP
haskellpattern-matchingelm
Mark Karavan
Mark Karavan

Reputation: 2674

Pattern Matching on a List in Elm

Can you pattern match on a list of items in Elm 0.18? For example:

type Thing = Foo | Bar | Baz

things : List Thing
things : [ Foo, Bar, Baz ]

caseStatement : Thing -> Bool
caseStatement thing =
  case thing of
    expressionSayingThatItIsInThingsList ->
      True
    _ ->
      False

Also, can this be done in Haskell?

Upvotes: 1

Views: 1084

Answers (2)

Mark Bolusmjak
Mark Bolusmjak

Reputation: 24399

Yes!

case thing of
   Foo :: Foo :: Bar :: [] ->
      "two foos and a bar"
   Bar :: stuff ->
      "a bar and then " ++ (toString stuff)
    _ ->
      "something else"

Upvotes: 1

Netwave
Netwave

Reputation: 42698

Elm is based on Haskell, actually has lot of less features, you can pattern match your type easily element by element or check if it is in the list:

data Thing = Foo | Bar | Baz deriving (Eq, Show)

things :: [Thing]
things = [ Foo, Bar, Baz ]

caseStatement :: Thing -> Bool
caseStatement thing = thing `elem` things

Pattern matching:

caseStatement :: Thing -> Bool
caseStatement Foo = True
caseStatement Bar = True
caseStatement Baz = True
caseStatement _   = False

Here you have a live example

In Elm you can use List.member

import List
type Thing = Foo | Bar | Baz

things : List Thing
things = [ Foo, Bar, Baz ]

caseStatement : Thing -> Bool
caseStatement thing = List.member thing things

Pattern matching it:

caseStatement : Thing -> Bool
caseStatement thing = case thing of
    Foo -> True
    Bar -> True
    Baz -> True
    _   -> False

Upvotes: 4

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