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Am 20.09.2011 22:55, schrieb Islon Scherer:
> Scala is a OO language with FP support, clojure is a functional
> language with OO support, they are very different. It's normal for
> someone with a OO background to think that every method receives a
> object of some kind and it's good to know it's type in advance, and
> if you want polymorphism you create a subclass or implement an
> interface but in clojure functions are polymorphic not classes.
> Let's take the function first as an example, if you give a String
> to first you'll get the first char, if you give a vector or list
> you'll get the first element, if you give a java array you'll get
> the element at index 0.
yes, but you magically need to know
a) for which types does it work? if you give a byte to the function,
will you get an error, or its first bit? or its first char after its
been converted to a string?
b) if i want my data structure to support this, how do i have to do that?
both questions are answered by a signature. the java one would be via
an interface, and scala offers structural types which are much more
elegant in this case:
def first[A](fromThis: {def first: A}) = fromThis.first
if that hurts, you can extract the type definition:
type SupportsFirst[A] = {
def first:A
}
def first[A](fromThis: SupportsFirst) = fromThis.first
the compiler will tell you if whatever you want to put in there
doesn't have a method "first" having a matching signature
> In scala each collection class has a overriden first method so you
> can have a static and pre-defined return type, in clojure the first
> function is itself polymorphic so it doesn't make sense to say that
> the return type of first is Object because it depends on the
> parameter. You can code clojure's first function in scala but the
> parameter and return type would be Object, and you would have to
> typecast it anyway.
no, see above. i can tell scala (and even java!) to return whatever is
coming in
> (Maybe with scala's extremely fancy type system you can create a
> generic first function a la clojure but the type signature would
> make my eyes hurt :) With doc, source (the functions) and the repl,
> a static typing system would'n be that useful
>
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i have to object. a function signature in scala that takes something
(A) which contains something else (B) and returns such a something
else (B), for example first, would look like this:
def first[A, M[B]](fromThis:M[B]):A
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