database - How to effectively check if an entry is present using HDBC in haskell? -

like mentionned in question, have unique constraint on table in want insert data. have check if data present , insert if not.

that can't seem able do. here code:

import system.environment (getargs) import database.hdbc import database.hdbc.sqlite3 import text.regex.posix import qualified data.bytestring.char8 b  getfrom bstr =   bstr =~ "from:.+@.+\\.(fr|com).?" :: b.bytestring  getto bstr =   bstr =~ "to:.+@.+\\.(fr|com).?" :: b.bytestring  getsubject bstr =   bstr =~ "subject:.+" :: b.bytestring  main =   --[dbpath, rawmail] <- getargs   bstr <- b.getcontents   conn <- connectsqlite3 "bmsg.db"   let     sqnomdest  = tosql $ getto bstr     sqnomexped = tosql $ getfrom bstr   -- begin of problematic part    -- make query know if entry if present   qdbefore <- quickquery' conn "select d_id dest maildest ?" [sqnomdest]   qebefore <- quickquery' conn "select e_id exped mailexped ?" [sqnomexped]   -- if not insert , else return arbitrary int   case qdbefore of --my check wrong since snippet go return 0     [[sqlnull]] -> run conn "insert dest(maildest) values(?)" [sqnomdest] --unique on column constraint if ran alone on present raise exeption     _           -> return 0   case qebefore of     [[sqlnull]] -> run conn "insert exped(mailexped) values(?)" [sqnomexped] --same here constraint     _           -> return 0   commit conn   --end of problematic part    [[qd]] <- quickquery' conn "select d_id dest maildest ?" [sqnomdest]   [[qe]] <- quickquery' conn "select e_id exped mailexped ?" [sqnomexped]   run conn "insert mails(d_id, e_id, sujet, mail) values (?, ?, ?, ?)" [qd, qe, tosql $ getsubject bstr, tosql bstr]   commit conn   disconnect conn   -- create table dest -- ( -- d_id integer primary key, -- maildest varchar(64) not null unique -- );  -- create table exped -- ( -- e_id integer primary key, -- mailexped varchar(64) not null unique -- );  -- create table mails -- ( -- m_id    integer primary key, -- d_id    integer not null, -- e_id    integer not null, -- sujet   varchar(128), -- mail    text not null, -- constraint fk_dest foreign key (d_id) references dest(d_id), -- constraint fk_exped foreign key (e_id) references exped(e_id) -- ); 

there no error, compiles, patern match fail when give email not present.

if query has no results, empty list, not list 1 row , 1 null -- that's why case fails when insert new thing. failing case, match on empty list.