Excerpts from Erik de Castro Lopo's message of 2010-10-14 22:44:03 +0200:
> Erik de Castro Lopo wrote:
>
> > Python and Ruby do have a strict type system, its just that
> > the types are checked at run time. In these dynamically typed
> > languages, *every* value is an object that caries around with
> > it its type.
>
>
> From at least one message in this thread it seems that there
> is some confusion about type systems, in particular strict
> vs strong typing.
>
> Type systems can basically be mapped onto two axes;
>
> strong
> |
> |
> |
> dynamic ----------+---------- static
> |
> |
> |
> weak
>
> Strong type systems prevent you from doing things that do not make
> sense like adding an integer and a list. For instance, in Python,
>
> >>> x = 1
> >>> y = [ 1 , 2 ]
> >>> x + y
>
> gives an error like "unsupported operand type(s) for +: 'int' and 'list'".
>
> Weak type systems allow things that don't make sense (possibly with
> a user provided type cast). For instance in C:
>
> float test (GLIst *list)
> { int x = 10 ;
> return x + (int) list ;
> }
>
> the above makes no sense whatsoever, but the compiler accepts the
> programmers word that this is in fact what the programmer wants.
>
> Purely static type systems are ones where all the types are checked at
> compile time and the run time objects contain no type information
> and no type checking is done at run time.
>
> Purely dyanmic type systems are ones where every run time data
> object carries around its type and all the type checking is done
> at run time.
>
> With the above axes set up you can have languages like C with
> weak static types, Haskell with strong static types and Python
> with strong dynamic types. I don't know of any language that
> can really be said to have weak dynamic types.
>
> Furthermore, most dynamically types languages have a small amount
> of static type checking built in, but this is usually a by product
> of syntax checking.
>
> Similarly, in a statically typed langauge its possible to implement
> a library with an object type where the type checking for operations
> on those objects is carried out at run time.
>
> Finally, many people assume that all strictly typed languages
> require the programmer to explicitly define the types of everything.
> Fortunately, this is not the case. There are a whole family of
> languages that use Hindley-Milner type inference to infer the types
> of all objects from the context. These languages include Ocaml and
> Haskell.
>
>
> HTH,
> Erik
Thanks for this clarification Erik, it's a topic lots of people are
confused about, if they have any idea at all. Your explanation with the
axis makes sense, thanks.
Philipp
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