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Need help - Syntax error at or near select

I am trying to create a new function (search function), but I get syntax error at or near "select" and I don't know why... 

create or replace function searchCategories(recherche text)
returns setof categories as $$
  select * 
  from categories 
  where to_tsvector(titre_fr || ' ' || titre_cn) 
  @@ to_tsquery(recherche);
$$ language plpgsql;

try putting a begin and end before select and before $$

Would you happen to know why I get ' type " " does not exist' on some queries?

not without more info...  normally setof must exactly match the table structure, but with * on your select not sure why that would be an issue.