* Re: [Caml-list] Weird typing problem
@ 2002-05-21 23:48 Ryan Tarpine
0 siblings, 0 replies; 6+ messages in thread
From: Ryan Tarpine @ 2002-05-21 23:48 UTC (permalink / raw)
To: caml-list
>From: Ching-Tsun Chou To: pixel@mandrakesoft.com CC:
>caml-list@pauillac.inria.fr Subject: Re: [Caml-list] Weird typing problem
>Date: Tue, 21 May 2002 16:20:11 -0700 (PDT)
>
>Thanks! But I still fail to see why the compiler and the interpreter should
>behave differently.
>
>- Ching Tsun
Being only a beginner, I think it's because in the interpreter the type of
the function doesn't need to be "finalized" completely, while in the
compiler must *absolutely* have the exact type so it can generate code. In
the interpreter, it's fine for the type to be "refined" later on, because it
won't be accessed by any outside applications.
The compiler can't generate code for functions with arguments of type '_a
because that would require the object code to be modified when the function
is called for the first time. The code would need to be changed depending
on which program called it, since (list_last [2;3]) and (list_last
[2.0;3.0]) would have different effects (the first one would make the type
of list_last be ints to ints while the second floats to floats).
HTH,
Ryan Tarpine, rtarpine@hotmail.com
"To err is human, to compute divine. Trust your computer but not its
programmer."
- Morris Kingston
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Caml-list] Weird typing problem
@ 2002-05-22 0:14 Ryan Tarpine
0 siblings, 0 replies; 6+ messages in thread
From: Ryan Tarpine @ 2002-05-22 0:14 UTC (permalink / raw)
To: ctchou, caml-list
>From: Ching-Tsun Chou <ctchou@mipos2.intel.com>
>To: caml-list@inria.fr
>Subject: Re: [Caml-list] Weird typing problem
>Date: Tue, 21 May 2002 16:58:32 -0700 (PDT)
>
>Why can't the compiler just substitute 'a for '_a if '_a can't be
>resolved? What's wrong with that? Surely O'Caml can handle
>polymorphism.
>
>- Ching Tsun
>-------------------
See http://caml.inria.fr/FAQ/FAQ_EXPERT-eng.html#polymorphisme. In short,
(quoted from the FAQ)
When type-checking
let name = expr1 ...
The type of expr1 is generalized when expr1 is a function, an dentifier or a
constant. Otherwise the identifier name is not polymorphic (type variables
are not generalized).
...
The new rule implies that if expr1 is a function application, then the
identifier name is monomorphic
This is done because "a secure type system for Caml must forbid the
existence of polymorphic mutable values at run-time" and this was basically
the easiest way to do it. Read the FAQ for an example.
HTH,
Ryan Tarpine, rtarpine@hotmail.com
"To err is human, to compute divine. Trust your computer but not its
programmer."
- Morris Kingston
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Caml-list] Weird typing problem
[not found] <F143h1mIs1Dk3fqSUVd000021b5@hotmail.com>
@ 2002-05-21 23:58 ` Ching-Tsun Chou
0 siblings, 0 replies; 6+ messages in thread
From: Ching-Tsun Chou @ 2002-05-21 23:58 UTC (permalink / raw)
To: caml-list
Being only a beginner, I think it's because in the interpreter the type of
the function doesn't need to be "finalized" completely, while in the
compiler must *absolutely* have the exact type so it can generate code. In
the interpreter, it's fine for the type to be "refined" later on, because it
won't be accessed by any outside applications.
The compiler can't generate code for functions with arguments of type '_a
because that would require the object code to be modified when the function
is called for the first time. The code would need to be changed depending
on which program called it, since (list_last [2;3]) and (list_last
[2.0;3.0]) would have different effects (the first one would make the type
of list_last be ints to ints while the second floats to floats).
Why can't the compiler just substitute 'a for '_a if '_a can't be
resolved? What's wrong with that? Surely O'Caml can handle
polymorphism.
- Ching Tsun
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Caml-list] Weird typing problem
2002-05-21 23:03 ` Pixel
@ 2002-05-21 23:20 ` Ching-Tsun Chou
0 siblings, 0 replies; 6+ messages in thread
From: Ching-Tsun Chou @ 2002-05-21 23:20 UTC (permalink / raw)
To: pixel; +Cc: caml-list
Thanks! But I still fail to see why the compiler and the interpreter
should behave differently.
- Ching Tsun
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From: Pixel <pixel@mandrakesoft.com>
Date: 22 May 2002 01:03:59 +0200
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Ching-Tsun Chou <ctchou@mipos2.intel.com> writes:
[...]
> let list_last = List.hd << List.rev
[...]
> val list_last : '_a list -> '_a = <fun>
make it:
let list_last l = (List.hd << List.rev) l
val list_last : 'a list -> 'a
that's monomorphism restriction:
http://caml.inria.fr/FAQ/FAQ_EXPERT-eng.html#variables_de_types_faibles
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Caml-list] Weird typing problem
2002-05-21 22:13 Ching-Tsun Chou
@ 2002-05-21 23:03 ` Pixel
2002-05-21 23:20 ` Ching-Tsun Chou
0 siblings, 1 reply; 6+ messages in thread
From: Pixel @ 2002-05-21 23:03 UTC (permalink / raw)
To: Ching-Tsun Chou; +Cc: caml-list
Ching-Tsun Chou <ctchou@mipos2.intel.com> writes:
[...]
> let list_last = List.hd << List.rev
[...]
> val list_last : '_a list -> '_a = <fun>
make it:
let list_last l = (List.hd << List.rev) l
val list_last : 'a list -> 'a
that's monomorphism restriction:
http://caml.inria.fr/FAQ/FAQ_EXPERT-eng.html#variables_de_types_faibles
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^ permalink raw reply [flat|nested] 6+ messages in thread
* [Caml-list] Weird typing problem
@ 2002-05-21 22:13 Ching-Tsun Chou
2002-05-21 23:03 ` Pixel
0 siblings, 1 reply; 6+ messages in thread
From: Ching-Tsun Chou @ 2002-05-21 22:13 UTC (permalink / raw)
To: caml-list
Consider a file "zzz.ml" that contains only two lines:
let (<<) f g x = f (g (x))
let list_last = List.hd << List.rev
Now, if I load the file interactively, I get:
<unix> ocaml
Objective Caml version 3.04
# #use "zzz.ml" ;;
val ( << ) : ('a -> 'b) -> ('c -> 'a) -> 'c -> 'b = <fun>
val list_last : '_a list -> '_a = <fun>
#
However, if I try to compile it, I get:
<unix> ocamlc -c zzz.ml
File "zzz.ml", line 2, characters 16-35:
The type of this expression, '_a list -> '_a,
contains type variables that cannot be generalized
Why so?
- Ching Tsun
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2002-05-21 23:58 ` Ching-Tsun Chou
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