Hello, I've a problem connecting to my postgresql database. Can You help me fix the ambigious type signature?
(The example is identical to the first 5-liner-example in the package documentation)
http://hackage.haskell.org/packages/archive/postgresql-simple/0.3.5.0/doc/html/Database-PostgreSQL-Simple.html Kind regards Hartmut ------------------------------------------------------------------------ {-# LANGUAGE OverloadedStrings #-} import Database.PostgreSQL.Simple main = do conn <- connect defaultConnectInfo query conn "select 2 + 2" return () ------------------------------------------------------------------------ But this leads to error: ------------------------------------------------------------------------ Line 9: 1 error(s), 0 warning(s) Couldn't match expected type `IO a0' with actual type `q0 -> IO [r0]' In the return type of a call of `query' Probable cause: `query' is applied to too few arguments In a stmt of a 'do' block: query conn "select 2 + 2" In the expression: do { conn <- connect defaultConnectInfo; query conn "select 2 + 2"; return () } ------------------------------------------------------------------------ OK, I see, that a parameter q is missing. I change the source code to {-# LANGUAGE OverloadedStrings #-} import Database.PostgreSQL.Simple main = do conn <- connect defaultConnectInfo query conn "select 2 + 2" ( ) {- added ( ) here -} return () ------------------------------------------------------------------------ Now, I run into next error: ------------------------------------------------------------------------ Line 9: 1 error(s), 0 warning(s) No instance for (FromRow r0) arising from a use of `query' The type variable `r0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance (FromField a, FromField b) => FromRow (a, b) -- Defined in `Database.PostgreSQL.Simple.FromRow' instance (FromField a, FromField b, FromField c) => FromRow (a, b, c) -- Defined in `Database.PostgreSQL.Simple.FromRow' instance (FromField a, FromField b, FromField c, FromField d) => FromRow (a, b, c, d) -- Defined in `Database.PostgreSQL.Simple.FromRow' ...plus 10 others In a stmt of a 'do' block: query conn "select 2 + 2" () In the expression: do { conn <- connect defaultConnectInfo; query conn "select 2 + 2" (); return () } In an equation for `main': main = do { conn <- connect defaultConnectInfo; query conn "select 2 + 2" (); return () } _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
