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+title = "Nominal Typing"
+author = "Yann Herklotz"
+tags = []
+categories = []
+backlinks = ["2a"]
+forwardlinks = ["2ab"]
+zettelid = "2a1"
++++
+
+Distinguish types by their names. For example, an `Integer` could be a
+`Count` or an `Offset`. Operations based on those types should interact
+in a different way. This is different to structured typing ([\#2ab]),
+which bases the difference of the type not on the name, but the
+structure. For example [^1]:
+
+-   `Count` + `Count` is valid, and should give a `Count`.
+-   `Offset` + `Count` is also likely valid, and will give an `Offset`,
+    because we can have an `Offset` and some count from a previous
+    value, and offset by the amount and the count of that object. The
+    new offset is likely to be valid as well.
+-   `Count` + `Offset` should probably not be valid, because it is maybe
+    expected that it returns a `Count`, which would however not be valid
+    (adding a count and an offset does not anything). However, to keep
+    associativity, it might make sense to allow this and return an
+    `Offset` in that case.
+-   `Offset` + `Offset` should not be valid, because this would allow us
+    to add offsets which may lead to a new offset that is out of range,
+    meaning we could access memory that is not valid.
+
+[^1]: Lelechenko, Andrew. *Haskell for mathematical libraries*. lambda
+    DAλS talk (13-14 Feb 2020). Kraków, Poland. \[Online\]
+    <https://www.youtube.com/watch?v=qaPdg0mZavM>.
+
+  [\#2ab]: /zettel/2ab