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points by cowlevel 2 days ago | 0 comments

In general it's a problem of sentinel values. Some languages let you abstract this - I think in Rust and C++ you can define an "intrusive optional" which is like optional but uses an invalid value pattern to store the not-present state. In memory it's the same as a sentinel value but the compiler forces you to unwrap it like an optional.

[–] eru 2 days ago

I'm not sure how well the 'forcing' works for Rust and C++. You can definitely do this kind of thing in dependently typed languages.

[–] cowlevel 1 day ago

The type is akin to optional<int>. You can't add 5 to it, you have to dereference it first which crashes if it was null (0 or another chosen value).

[–] eru 1 day ago

Well, in Rust you would use pattern matching, which can't even crash.

In Haskell, the closest thing to adding 5 to an optional value would be something like:

    fmap (5+) my_optional_value

This one leaves 'Nothing' alone, but adds 5 in case the value is present.