leftMap

leftMap is a function commonly found in functional programming libraries, particularly those dealing with data structures like `Either` or `Tuple`. Its primary purpose is to transform the "left" value of such a structure while leaving the "right" value unchanged. This is most often seen in the context of the `Either` type, which represents a value that can be one of two possible types, conventionally named `Left` and `Right`. The `Left` case typically signifies an error or failure, while the `Right` case represents a successful result.

When `leftMap` is applied to an `Either` value, it takes a function as an argument. If the `Either` is a `Left` value, the provided function is applied to the value contained within the `Left`. The result of this function becomes the new value of the `Left`. If the `Either` is a `Right` value, `leftMap` does nothing to the `Right` value; it is passed through unchanged. This behavior makes `leftMap` useful for error handling, allowing developers to modify or transform error information without affecting the successful outcomes.

The signature of `leftMap` typically looks like this: `leftMap<B>(f: (a: A) => B): Either<B, R>`. Here, `A` is the original type of the left value, `B` is the new type of the left value after transformation, and `R` is the type of the right value, which remains constant. This functional approach promotes immutability and predictable program flow.