undefined | Better HN

0 pointsvutekst11y ago0 comments

> Even in Haskell values must be tagged with types, and the type tags are inspected at runtime.

This does not fit my understanding of pattern matching in Haskell. Say you are matching over a value of type `Maybe String`. The type of the value is always `Maybe String`, it's just that its value might be `Just "foo"` or `Nothing :: Maybe String`. It is not to my knowledge tagged with type information at runtime in a compiled program, merely value information. The different values an ADT disjunction can take on all have the same type as each other.

> Otherwise, pattern matching wouldn't work (pattern matching is always based on type reflection).

What about pattern matching a String against a series of literal values? I don't see how this is based on type reflection, merely inspection of values.

0 comments

tel11y ago

In this context "tag" refers to the part of a value of type Maybe a which indicates whether that actual value is of the form Just a or Nothing. In other words, it's entirely runtime information distinguishing between various members of the same type

vutekstOP11y ago

This is my understanding as well, and that's why I objected to the parent post's assertion that "even in Haskell values must be tagged with types, and the type tags are inspected at runtime". The enum/ADT discriminator tag is value information rather than type information.

pron11y ago

Just and Nothing are different types even if the Haskell nomenclature doesn't call them so. The word "type" in Haskell means something different than it does in other languages. Haskell uses type to mean the mathematical notion of a set, while in CS, type usually refers to data memory layout. When comparing Haskell to other languages, we can't confuse terminology. In OO terms, for example, Just and Nothing are subtypes of Maybe, so the tag differentiating the two is exactly the same as that used to verify downcasts from the supertype Maybe to its subtypes, in, say, C++ or Java. Haskell implements the very same mechanism (call it reflection or RTTI).

tel11y ago

Meh, that's a way to talk about it but the standard type system of Haskell doesn't include subtyping like that. It's possibly you could apply a subtyping analysis to Haskell, but it's certainly non-standard and I'm not sure what you really, literally gain.

1 more reply

j / k navigate · click thread line to collapse

0 comments

tel11y ago

vutekstOP11y ago

pron11y ago

tel11y ago

1 more reply

j / k navigate · click thread line to collapse