Maps in PureScript

Our first program will explore the usage of maps, which are key-value data structures. Here’s the full source code translated into PureScript.

module Main where

import Prelude
import Data.Map as Map
import Effect (Effect)
import Effect.Class.Console (log)

main :: Effect Unit
main = do
    -- Create an empty map
    let m = Map.empty :: Map String Int

    -- Set key/value pairs
    let m1 = Map.insert "k1" 7 m
    let m2 = Map.insert "k2" 13 m1

    -- Print the map
    log $ "map: " <> show m2

    -- Get value for a key
    let v1 = Map.lookup "k1" m2
    log $ "v1: " <> show v1

    -- Get value for a non-existent key (returns Nothing)
    let v3 = Map.lookup "k3" m2
    log $ "v3: " <> show v3

    -- Get the size of the map
    log $ "len: " <> show (Map.size m2)

    -- Delete a key/value pair
    let m3 = Map.delete "k2" m2
    log $ "map: " <> show m3

    -- Clear the map (keep as empty)
    let m4 = Map.empty
    log $ "map: " <> show m4

    -- Check if a key is present
    let isKeyPresent = Map.member "k2" m2
    log $ "prs: " <> show isKeyPresent

    -- Declare and initialize a new map
    let n = Map.fromFoldable [("foo", 1), ("bar", 2)] :: Map String Int
    log $ "map: " <> show n

    -- Utility function to compare maps
    let n2 = Map.fromFoldable [("foo", 1), ("bar", 2)] :: Map String Int
    if n == n2 then log "n == n2" else pure unit

Explanation

Creating an Empty Map: We use Map.empty to create an empty map of type Map String Int.
Setting Key/Value Pairs: Use Map.insert to add key-value pairs to the map.
Printing a Map: log and show are used to print the map in a readable format.
Getting a Value: Map.lookup is used to retrieve the value associated with a particular key.
Handling Non-existent Keys: If a key doesn’t exist, Map.lookup returns Nothing.
Getting the Size of the Map: Use Map.size to obtain the number of key-value pairs.
Deleting a Key/Value Pair: The Map.delete function removes a specified key from the map.
Clearing the Map: Set the map to Map.empty to clear all key-value pairs.
Checking Key Presence: Use Map.member to check if a key exists in the map.
Declaring and Initializing a Map: Map.fromFoldable initializes a new map with given key-value pairs.
Comparing Maps: Directly compare two maps to check for equality and log the result.

Now that we have explored map functionalities, let’s dive deeper into other aspects of PureScript.

Slices in PureScript

Functions in PureScript

Learn X By Example

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Maps in PureScript

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Maps in PureScript

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