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Why doesn't ocamlopt detect a missing ; after failwith statement?
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Date: -- (:)
From: Brian Hurt <bhurt@s...>
Subject: Re: [Caml-list] Why doesn't ocamlopt detect a missing ; after failwith statement?
On 26 Nov 2004, skaller wrote:

> On Fri, 2004-11-26 at 08:14, Nicolas Cannasse wrote:
> > > All well and good, but I don't understand why it doesn't warn me about
> > > the missing ';' in the first case.
> > 
> > val failwith : string -> 'a
> > 
> > so failwith "error" prerr_endline "OK";
> > 
> > is a valid call since 'a unify with (string -> unit) -> string -> unit
> 
> .. a problem which could not occur were there a void type
> which couldn't unify with 'a, and prerr_endline had
> type string-> void.
> 
> 

There is one- it's called unit.  And prerr_endline probably already uses
it. The problem isn't with prerr_endline, the "problem" is with failwith.

failwith needs to return 'a, as it doesn't return.  If it returned some 
other type, I couldn't write code like:
    if some_test then
        failwith "some_test"
    else
        some_value

To make the above expression type correctly, failwith has to return the 
same type as some_value- which could be anything.  Therefor, failwith 
needs to return 'a, a value which can unify with (be the same type as) 
anything else.

The next problem comes in how Ocaml decides when and to what to apply 
arguments.  Consider the expression:
    f [1;2;3]
Fairly obvious, right?  We're calling f with an argument of an int list.  
Not necessarily.  Consider:
    List.map f [1;2;3]
Now f, instead of being the function we're passing arguments into, is now 
an argument itself.

So now, this is exactly the problem we're running into- prerr_endling is 
being treated exactly like f above- one minor change, and it's getting 
turned into an argument when it's meant to be a function call.

Does this help?

Brian