On Sun, 20 Jun 2010 16:07:08 +0200
Jürgen Hestermann <juergen.hesterm...@gmx.de> wrote:
> e:)
> > * What is type checking _actually_ useful for?
>
> I would be lost without type checking,
> especially when using sophisticated data structures
> like pointers to arrays of records (which again
> contain pointers to other complex structures).
> An example:
>
> type ArrayType = array[1..10] of SomeType;
> PointType = ^ArrayType;
> var X : PointType;
> Y : SomeType;
>
> What happens if you forget a dereferencing ^ in an expression like this:
>
> Y := X[7]
>
> where it should have been
>
> Y := X^[7]
For sure!
Now, it is easy for the compiler to guess such cases. Some languages perform
automagic deferencing of pointers to structured types (record/object, array),
meaning eg
p.x
is interpreted as
p^.x
and
p[i]
is interpreted as
p^[i]
There is no ambiguity since the pointer nature of p is known by the compiler
before such lookups can be parsed.
Oberon -- also designed by Wirth and thus not to be considered as weakly
secured ;-) -- works that way. Also note that all languages holding implicitely
referenced object types can only work that way... including fp for Delphi-like
OO (and all dynamic languages like python or Lua).
> What does the compiler do without type checking
> in the first case? Does it access X as if it
> was an array (of what element type/size?).
>
> It often happens that I change a type into a
> pointer to that type. Then I expect that the
> compiler shows me all places where I now get
> a syntax error so that I can change the code
> instead of having the compiler "guessing"
> some meaning (that I may not have intended).
Nice use of the compiler ;-) But only needed since implicite dereferencing is
not a feature of the language.
> I also often change a string type into an integer
> and vice versa. That implies a lot of code changes.
> Again with type checking I can lean back and
> rely on that the compiler will show me all places
> where these changes are needed.
Denis
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