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Date: 2008-01-05 (16:41)
From: Dario Teixeira <darioteixeira@y...>
Subject: Type_of?

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 |
        | `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 |
        | `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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