This is a good discussion of a problem that suits Haskell well, but it's unfair to Python in some respects. For example: I haven't read or written a lot of Pyt

This is a good discussion of a problem that suits Haskell well, but it's unfair to Python in some respects. For example: I haven't read or written a lot of Python in a while, but would Python programmers really want to implement mutation in such a class by copying dicts around? The hand-wringing about "oh no, I wrote the condition as not is_node" is silly since one could just define an is_leaf method that can be used without negation. And "changing heapToList to return a lazy sequence makes it no longer return a list, oh no!" is just as silly, since one would of course not do that but define a separate heapToSequence (and probably base heapToList on that). Also: "pattern matching and algebraic data types which have massive, clear benefits and few (if any) downsides". They have downsides whenever your data is not tree-shaped. Yes, a lot of data is tree, but then a lot isn't. I work in compilers, which is often touted as a prime application of ML-family languages, and this is very true, but not 100%. If you can have sharing in your abstract syntax tree (like a goto node that might refer to a target node somewhere else in the tree), you start to have difficulties. And you get even more difficulties when you try to model control flow graphs and the like. Nothing insurmountable, but still things where it's suddenly good to have other tools than only algebraic datatypes. OCaml is good in this regard.