Re: [m-users.] Impurity needed?

Volker Wysk <[email protected]> Mon, 13 Feb 2023 14:24:52 +0100

Newsgroups	gmane.comp.lang.mercury.general
Message-ID	<[email protected]>

Hi again.

The type of the odbc.transaction predicate is:

:- pred odbc.transaction(data_source::in, user_name::in, password::in,
     transaction(T)::transaction, odbc.result(T)::out, io::di, io::uo) 
is det.


Would it be possible to just change the transaction type from this:

:- type odbc.transaction(T) ==  pred(T, odbc.state, odbc.state).
:- mode odbc.transaction ==  (pred(out, di, uo) is det).


To this:


:- type odbc.transaction(T) ==  
     pred(T, odbc.state, odbc.state, io.state, io.state).
:- mode odbc.transaction ==  (pred(out, di, uo, di, uo) is det).


So the transaction predicate can do IO, when called. This looks rather easy,
but I'm not sure I've understood correctly.

Volker

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