this post was submitted on 14 Aug 2023
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"Some generic class" with specific methods and laws, Monads are an algebraic structure and you want those laws included same as if you enable some type to use
+
you want to have a0
somewhere andx + 0 == x
to hold. Like"foo" + "" == "foo"
in the case of strings, just as an example.In Rust,
Result
andOption
actually are monads. Let's takeOption
as example:pure x
isSome(x)
a >>= b
isa.and_then(b)
Then we have:
Some(x).and_then(f)
≡f(x)
x.and_then(Some)
≡x
m.and_then(g).and_then(h)
≡m.and_then(|x| g(x).and_then(h))
Why those laws? Because following them avoids surprises like
x + 0 /= x
.Rust's type system isn't powerful enough to have a Monad trait (lack of HKTs) hence why you can't write code that works with any type that implements that kind of interface.
Result
names>>=
and_then
, just likeOption
does so the code reads the same but you'll have to choose betweenOption
orResult
in the type signature, the code can't be properly generic over it.