AW: [Caml-list] some beautiful OCaml code

[Gerd Stolpmann <info@gerd-stolpmann.de>]

Am 09.01.2013 09:38:51 schrieb(en) David MENTRE:
> Hello OCaml experts,
> 
> 2013/1/9 Francois Berenger <berenger@riken.jp>:
> > There is a full blog post about it there:
> >
> >  
> http://functional-orbitz.blogspot.se/2013/01/introduction-to-resultt-vs-exceptions.html
> 
> Regarding this blog post, the final code is using Polymorphic Variants
> (http://caml.inria.fr/pub/docs/manual-ocaml/manual006.html#toc36).
> E.g.
> """
>     | _ ->
>       Error (`Bad_line s)
> """
> 
> I never fully grasped polymorphic variants compared to regular ones
> but I always had the feeling the polymorphic variants where less safe
> that variants because they would allow more possibility to mix
> unrelated things[1].

That's exactly the point: Polyvariants allow you to mix unrelated  
things. I don't think, though, that they are unsafer, because you get  
help from the compiler to keep these things nevertheless separated. You  
need to be aware what can happen, and here and there it is helpful to  
add a type hint or a coercion to control typing.

> Are the use of polymorphic variant mandatory to write code
> Return-Value-style code or can regular variants be used?

No. You can also use normal variants, but of course you need then a  
declaration like

type error = Bad_line of string | Bad_name of string | ...

before using it. Also, function composition can be very painful.  
Imagine you wrote two modules M1 and M2 in this style, and each module  
defines an error type. Now you want to call functions from both modules  
at one place, and run into the problem that you get [Error e1] values  
with [e1:M1.error_type] and [Error e2] values with [e2:M2.error_type].  
There is no simple way to mix that. Probably you need to do

type error = M1_error of M1.error | M2_error of M2.error

which is kind of inelegant. Polyvariants, in contrast, allow you to  
directly unite the error types as long as the tags are not  
contradictory. This means function composition as again as easy as if  
you used exceptions for error reporting.

Gerd

> Best regards,
> david
> 
> [1] Of course this ability is the very thing that is of interest to
> people using polymorphic variants.
> 
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> 

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Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
Creator of GODI and camlcity.org.
Contact details:        http://www.camlcity.org/contact.html
Company homepage:       http://www.gerd-stolpmann.de
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