From: Richard Jones <rich@annexia.org>
> > Why this error (below)?  I can "fix" it by adding quite a lot of
> > explicit types (I need one on every function which uses the
> > Dbi.connection parameter).  Is there a simpler way to fix it?

If I look at the details of this error, I can see:
   map : ([> `String of 'c ] list -> 'c) -> string list;
in the first type, and
   map : 'g. (Dbi.sql_t list -> 'g) -> 'g list;
in the Dbi.statement part of the second type.
These two types are incompatible: a polymorphic method type can only
be unified with a method with the same polymorphism.

Before calling a polymorphic method, you must make sure that its type
is known to the compiler, otherwise you get the above kind of errors.
The local solution is to annotate every polymorphic method call with
the type of the object:
   (stm : Dbi.statement)#map ...
But the system is more clever than that: you can simply annotate all
parameters which may eventually yield a Dbi.statement, whenever this
Dbi.statement is used for a polymorphic method call.
This is probably why you found that annotating all Dbi.connection
parameters worked. But you don't need to annotate all of them, just
the ones where there is a polymorphic method call in the body.
Note also that you can annotate with #Dbi.connection, to allow
subclasses.

(Actually, annotating all of them may be a good idea. Olabl had a
warning had warning when calling a method on an object of unknown
type.)

Jacques Garrigue

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