Assign value if not exist in Haskell

In Haskell, you can assign a value to a variable if it does not exist by using the maybe function. The maybe function takes two arguments: a default value and a Maybe value. If the Maybe value is Just, then the function returns the value inside the Just constructor. If the Maybe value is Nothing, then the default value is returned.

Here is an example of how to use maybe to assign a value if it does not exist:

maybeValue = Nothing
defaultValue = 42
newValue = maybe defaultValue id maybeValue

In this example, maybeValue is set to Nothing, meaning there is no value assigned to it. defaultValue is set to 42, which we want to assign to newValue if maybeValue has no value.

We then use the maybe function to assign maybeValue to newValue, using defaultValue as the default value. In this case, id is used as the second argument to maybe, which is just a function that returns its input unchanged.

Since maybeValue is Nothing, the default value of 42 is returned and assigned to newValue.

However, if maybeValue had a value of, say, Just 10, then 10 would be returned instead and assigned to newValue.