[lmkg]

Clojure is dynamically typed in the sense that programs are not type-checked at compile time, values are tagged with types, and type errors are runtime errors. On other words, it is dynamic, in contrast to static type systems. This says nothing about strong vs weak typing, nor expressiveness of the type system, nor type-dependent semantics like polymorphism.

Personally I find it less confusing that dynamic typing is a small and comparatively well-defined concept, than mixing it in with aspects of polymorphism and dynamic dispatch. Dynamic dispatch doesn't seem to me to have much to do with dynamic typing; it's a fundamental tool of expression in C++ and Java, and those are both statically-typed languages.

I'm curious why you think multimethods are more limited than dynamic dispatch. It seems to me that their expressiveness is a strict superset of dynamic dispatch. Am I missing something?

[pron]

> Clojure is dynamically typed in the sense that programs are not type-checked at compile time, values are tagged with types, and type errors are runtime errors.

Well, yes, but that has little to do with the way Clojure abstractions work; namely, they're not based on dynamic dispatch, but on OO-style polymorphism.

But BTW, your definition is not entirely complete, and the distinction between statically typed and dynamically typed is not always so clear cut when you're not talking about extremes such as Haskell and JavaScript.

Statically typed languages like Java, C++, C# and Scala can, and do have runtime type errors because they allow casting. They also all have type tags (well, optional in C++). I.e., most statically typed languages don't attempt to eliminate all type errors at compile time. Clojure indeed does very little static type checks (I think only function arity is checked).

Even in Haskell values must be tagged with types, and the type tags are inspected at runtime. Otherwise, pattern matching wouldn't work (pattern matching is always based on type reflection).

> I'm curious why you think multimethods are more limited than dynamic dispatch.

Because methods can't (or rarely do) "appear out of nowhere" or get installed at runtime as they do in, say, JavaScript or Groovy.

[vutekst]

> 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.

[tel]

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

[vutekst]

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.

[pron]

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).

[tel]

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.

[willismichael]

"can't" is a far cry from "rarely". Multimethods can be dynamically created during runtime.

[pron]

Multimethods in general are not very common in Clojure. My point is that in order to understand Clojure you shouldn't think about it as you do about most dynamic languages, because its core mechanisms and abstractions are based on OO interfaces rather than dynamic dispatch.

[adrianm]

You are very wrong about this. Clojure libraries are a very diverse ecosystem, and even a cursory analysis of the most widely used libraries will prove your assertion wrong. Heck, you can't even make custom printers (for your own types) in Clojure without extending the print-method multimethod.

Please don't speak for the Clojure community as a whole, because it spreads misinformation about both the libraries and the language itself. If you want to relate your personal experiences, fine, but your original post and this both convey a sense of absolutism in your characterization about Clojure's features that are just not true.