Web Development with ReasonML

Using Belt.Map.update

Back in our discussion of  Belt.Map, we updated a map of the color counts by using Belt.Map.getWithDefault and Belt.Map.set. While this works, you may find it more convenient to use Belt.Map.update. This function takes as its parameters the map to be updated, a key, and a function. It works as follows:

• Attempt to get the value for the given key using Belt.Map.get. This returns an option. If the value is None, the key isn’t in the map, so return the map unchanged.

• Otherwise, apply the function to the value. If the function returns None, return a map with the key and value removed.

• If the function returns Some(value), then return an updated map where the value for the given key is value.

Let’s look at a concrete example: ...