I have to be kind of dumb, but watching all blogs and documentation https://hackage.haskell.org/package/containers-0.4.2.0/docs/Data-Map.html
I cannot figure how to create a simple Map collection with a simple key -> value.
Sorry I´m very newby.
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1This might be useful for you: haskell-containers.readthedocs.io/en/latest.sjakobi– sjakobi2018-05-12 09:27:17 +00:00Commented May 12, 2018 at 9:27
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2 Answers
you should look for functions with signature like a -> b -> ... -> Map k v, where none of a,b... (the input parameters) is a Map. From Data.Map such are empty, which creates empty map, singleton, which creates map with one element, and all flavours of from*List*
1 Comment
paul
I found Singleton which I think is the same than a fromList with one element. But well now is pretty clear the API. Thanks
First, make sure you have containers package installed via cabal or stack. I am assuming you want to use the latest version of the package (0.5.11.0). The link on hackage that you posted points to very old version of the package.
Then import the Data.Map
import qualified Data.Map.Strict as Map
import Data.Map (Map())
Use fromList function to create a map
-- map with ints as keys and strings as values
myMap :: Map Int String
myMap = Map.fromList [(5,"a"), (3,"b"), (5, "c")]
If you want to cut the boilerplate, you can use OverloadedLists extension.
-- put extensions at the top of your file
{-# LANGUAGE OverloadedLists #-}
import qualified Data.Map.Strict as Map
import Data.Map (Map())
-- map with ints as keys and strings as values
myMap :: Map Int String
myMap = [(5,"a"), (3,"b"), (5, "c")]
4 Comments
paul
I just saw the fromList operator, but I was wondering if it was a simple way without have to create a list. Still is what I´m doing already, thanks a lot for the answer
Erik Cupal
Unfortunately, the syntax is little bit verbose. You can make it a little bit less verbose by enabling
OverloadedLists extension. You can then omit calling fromList. I'll add it to my answer.paul
myMap = [(5,"a"), (3,"b"), (5, "c")] how is possible that this is not getting conflict with a List of tuple of (Int, String) ??!
Erik Cupal
Because myMap is explicitly typed as Map Int String. If the type can be derived from context (from usage), it can be omitted.