Hello,

How about giving it a name ?

For instance:

type 'a make_doc_type = 'a XHTML.M.elt list
   constraint 'a =  [< `Address | `Blockquote | `Del | `Div | `Dl | 
`Fieldset | `Form | `H1 | `H2 | `H3 | `H4 | `H5 | `H6 | `Hr | `Ins | 
`Noscript | `Ol | `P | `PCDATA | `Pre | `Script | `Table | `Ul > 
`Blockquote `H1 `P ]

(this is untested, might contain a few dozens syntax errors, but it's 
the idea).



Arnaud Spiwack

Dario Teixeira a écrit :
> Hi,
>
> I'm using the XHTML.M module from the Ocsigen project to generate valid
> XHTML pages.  It makes heavy use of polymorphic variants, and as such the
> types produced can be quite verbose and complex.  In practice, however, one
> is rarely confronted with them (thank goodness for latent typing and type
> inference!).  There is one exception, though: when unmarshalling, one must
> explicitly provide the return type.  And this is the context for my question.
>
> So, imagine I have a module with only two functions.  The first, "make_doc",
> uses XHTML.M and its signature is therefore quite complex.  This is what
> "ocamlc -i" tells me:
>
> val make_doc: string ->
>         [< `Address | `Blockquote | `Del | `Div | `Dl | `Fieldset | `Form | `H1
>         | `H2 | `H3 | `H4 | `H5 | `H6 | `Hr | `Ins | `Noscript | `Ol | `P |
> `PCDATA
>         | `Pre | `Script | `Table | `Ul > `Blockquote `H1 `P ]
>         XHTML.M.elt list
>
>
> The second function, "unpickle_doc", uses the Marshal module to deserialise
> from a string a previously pickled doc.  This is the definition of this
> function:  (note that I've used copy & paste of the previous output of
> "ocamlc -i" to provide the explicit type annotation)
>
> let unpickle_doc str :
>         [< `Address | `Blockquote | `Del | `Div | `Dl | `Fieldset | `Form | `H1
>         | `H2 | `H3 | `H4 | `H5 | `H6 | `Hr | `Ins | `Noscript | `Ol | `P |
> `PCDATA
>         | `Pre | `Script | `Table | `Ul > `Blockquote `H1 `P ]
>         XHTML.M.elt list
>         = Marshal.from_string str 0
>
>
> Now, this works fine.  It is however error prone, since if the signature
> of make_doc changes somewhat and I forget to update the type annotation in
> unpickle_doc, then I get a nasty runtime segfault.  So this is my question:
> since the return type of make_doc is known at compile-time, is there any
> way I can tell the compiler that the return type of unpickle_doc should be
> the same as make_doc's?
>
> Thank you for your help!
> Dario Teixeira
>
>
>
>
>
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