Hello, the MonadFix wiki at https://wiki.haskell.org/MonadFix has a statement that I feel is a bit misleading. In section "2.2 Lazy algorithm interleaved with effects", it claims that making the BTree data structure strict doesn't cause endless recursion. Well, that's true, but that's just because rep_x_sum returns a tuple containing the BTree and the summed values of the current subtree, and the tuple is lazily constructed - postponing the construction of the tree value. So highlighting the fact that the function still works when the BTree structure is made strict is kind of a red herring. Maybe the confusion could be avoided by removing the part about making BTree strict, or adding a note about the tuple still ensuring lazy construction? -- Samuel