Adding pseudo columns to a projection is cumbersome, to say the list. For example, to add a field named "hidden" to a query I have to write: data Hide = Hide instance FieldTag Hide where fieldName _ = "hide" hideField = mkAttr Hide I wrote a little Template haskell that reduces this to: $(mkField "hide") If you want to customize the name of the column in the generated SQL, another function is used: $(mkFieldWithName "NumPullingParts" "num_pulling_parts") The code is below. Some questions: * Does the code look reasonable and useful? * Would the haskelldb maintainers want to introduce a dependency on template haskell to haskelldb for this kind of code? * Otherwise, would an "extensions" package make sense? If the reaction is positive, I'll work on extending this for a patch. My heartfelt thanks to Bulat for his excellent template haskell tutorials. I really wish that library had better documentation! Justin mkField :: String -> Q [Dec] mkField [] = error "Can't generate field from an empty string." mkField f = let fieldType = if isUpper (head f) then f else (toUpper (head f)) : tail f colName = if isLower (head f) then f else (toLower (head f)) : tail f in mkFieldWithName fieldType colName -- ^ Creates a compile time field declaration using the given -- arguments for the type and column name of the field. The fieldType argument -- is used to produce the "<field name>Field" function which can be used to -- add the field to a projection. -- -- Note that an error will occur if fieldType is not a proper type/constructor name. mkFieldWithName :: String -> String -> Q [Dec] mkFieldWithName [] _ = error "Can't create a field with an empty type." mkFieldWithName _ [] = error "Can't create a field with an empty column name." mkFieldWithName fieldType colName = let ty = mkName fieldType fieldName = toLower (head fieldType) : tail fieldType fieldD = [DataD [] ty [] [NormalC ty []] []] fieldI = [InstanceD [] (AppT (ConT (mkName "Database.HaskellDB.HDBRec.FieldTag")) (ConT ty)) [FunD (mkName "fieldName") [Clause [WildP] (NormalB (LitE (StringL colName))) []]]] fieldF = [ValD (VarP (mkName (fieldName ++ "Field"))) (NormalB (AppE (VarE (mkName "Database.HaskellDB.DBLayout.mkAttr")) (ConE ty))) []] in return (fieldD ++ fieldI ++ fieldF)