Comments on: Some interesting features of Haskell’s type system

By: AdelNick

AdelNick — Fri, 04 Mar 2016 09:19:05 +0000

Ah, indeed. Pretty simple. Thank you for the explanation!

By: Wolfgang Jeltsch

Wolfgang Jeltsch — Thu, 03 Mar 2016 22:01:39 +0000

According to the type declarations, every tree contains a forest, but a forest contains something of type func (Tree func label). There may be values of this type that do not contain trees. For example in the case of RoseTree, func is [], so func (Tree func label) is [RoseTree label]. As a result, you have a base in the form of the empty list. For instance, the expression Tree 'X' (Forest []) denotes a rose tree with just a single node with label 'X'. Note that even without such a base, the Tree and Forest types are not useless, since you can have infinite data structures.

By: AdelNick

AdelNick — Thu, 03 Mar 2016 00:10:50 +0000

Thank you for the interesting post. One question I couldn’t answer for myself: How do I create instances of types Tree and Forest?
For example, later when you define type List, there is a non-recursive constructor Nil, that allows to stop recursion. In case of Tree and Forest, the recursion seems to be infinite.

By: Some interesting features of Haskell’s type system | Theory Lunch

Some interesting features of Haskell’s type system | Theory Lunch — Sat, 09 Feb 2013 22:01:08 +0000

[…] I talked about some of Haskell’s type system features. You can find a write-up of my talk on my personal blog. […]