Matching fun M:F/A

Jesper Louis Andersen jesper.louis.andersen@REDACTED
Thu Jan 2 13:40:55 CET 2020

On Tue, Dec 31, 2019 at 5:12 PM Pierre Fenoll <pierrefenoll@REDACTED>

> Thoughts?
Admitting equality and by extension unification on function values is a
dangerous game which you usually don't want to play. The reason is that
equality often has to be tailored to the situation.

Stake #1 in the vampire: Function equality is undecidable[0]. We can't
generally handle this:

F = fun(X) -> X + 3 end,
G = fun(X) -> X + 2 + 1 end,

though an optimizing compiler might decide to turn G into the exact same
byte code instructions as F.

Stake #2 in the vampire: reference equality is not very Erlang-like, but it
gives you *some* equality over the function domain. We tend to test
equality by value only.

Stake #3 in the vampire: sensible languages outright *reject* equality on
certain types. And by extension, not all types can be compared in the usual

Stake #4 in the vampire: you often want to equip several different types of
equality to a type[1]

[0] Sketch of proof: reduce to a solution to the halting problem. Also see
Rice's theorem.
[1] Classic Haskell98 has a couple of weaknesses here. If you implement Eq,
then you cannot easily implement another kind of Eq for the same type
unless you newtype-wrap it. It is also prevalent in the Ord class with

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