[sixbrx]

Why would you need to change its parts, it can be parameterized over the parts that change, by passing in other functions as arguments. It's also cheap conceptually and syntactically to create new ones, if an existing one doesn't fit the bill.

[zby]

I don't really know - I've never extensively programmed in functional way (beside studies) - but the point about overriding is that you do it post-hoc you get a library class and you change it. The original author does not need to imagine all the ways you'd like to modify it - and yet by doing some basic structural design and putting stuff into methods he gives you the opportunity to override. Also I imagine that this level of parametrizing - that is if every function in a package is a parameter to another function in that package then it gets messy, but with subclassing/overriding you can change any method and it changes for all other methods.