Artyom Shalkhakov wrote:
Now, in the manual an SQL table is said to be introduced by a
type-level function:
con sql_table :: {Type} -> {{Unit}} -> Type
(I read it thus: "give me a schema, and I promise to give you a type
of your table")
The question is: is it possible to get the set of attributes and the
set of keys given a table?
No. In general, there is no way to extract the indices of a type.
Instead, you must pass the indices as separate arguments. I believe
this is shown clearly by the Crud demo's functor signature.
In the orm you mentioned, the facility of defining tables and foreign
key relationships between them essentially duplicates (part of) that
of the language (why? is there anything I'm missing?), giving you neat
functions in exchange.
It was just an example of a nifty thing you can do. The whole thing
makes just as much sense when the functor takes a table as a new input.
What if I already have the tables defined and now I just want a few
(effectful) frequently needed functions? Then, given a table, it's
attributes and the primary key, we can do our job.
Crud does exactly this. Can you explain which new thing you want to do,
such that it is not demonstrated in Crud? (The fact that the functions
are local [not exported in the functor's output signature] and/or the
fact that they generate web pages is not relevant to the functor's input
signature.)
Thinking this way, I write a module with a signature like this:
functor Entity(M: sig
table t (* the parameter *)
end): sig
val create: row' -> transaction id (* FIXME: how to deduce [row'] and [id]
from [t]? *)
(* other functions *)
end
The table [t] must have a primary key [id: int] (or something, we
don't want to dictate naming), and other fields we don't really care
about.
You can't declare a table input without declaring its column types.
Again, I'm confused about how to respond, since I believe Crud
demonstrates exactly this pattern, even up to the required presence of
an id column.
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