Maxima per row

I have a table, one of whose fields indicates the date from which
information in the row becomes valid, and another indicates the type to
which the row refers. How do I design a query such that I get one row for
each type, that row being the most recent info about that type? I.e. what I
want is

CREATE TABLE info (id serial primary key,typeid int4,validfrom
date,description text);
INSERT INTO info (typeid,validfrom,description) VALUES (1,'01/01/1999','a');
INSERT INTO info (typeid,validfrom,description) VALUES (1,'01/01/2000','b');
INSERT INTO info (typeid,validfrom,description) VALUES (2,'01/01/1998','c');
INSERT INTO info (typeid,validfrom,description) VALUES (2,'01/01/1999','d');
INSERT INTO info (typeid,validfrom,description) VALUES (2,'01/01/1997','e');

SELECT typeid,validfrom FROM prices GROUP BY typeid HAVING
validfrom=MAX(validfrom);

but of course the last is illegal. The result I'm looking for is

1,'01/01/2000','b',
2,'01/01/1999','d'

Thanks,
Moray

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Moray(dot)McConnachie(at)computing-services(dot)oxford(dot)ac(dot)uk