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sqlite-simple-errors
Hackage
Light wrapper around errors from sqlite-simple to make working with constraint errors a bit easier.
The wrapper function, runDBAction allows you to run your database code and get the errors returned to you:
type DatabaseResponse a = Either SQLiteResponse a
runDBAction :: IO a -> IO (DatabaseResponse a)
SQLiteResponse encapsulates the different error types that can be thrown by sqlite-simple, as well as providing a new error type for constraint violations that is more immediately actionable:
data Constraint = NotNull
| ForeignKey
| Unique
| Check
deriving (Show, Eq)
data SQLiteResponse = SQLConstraintError Constraint Text
| SQLFormatError FormatError
| SQLResultError ResultError
| SQLOtherError SQLError
deriving (Show, Eq, Typeable)
Usage
Let's use sqlite-simple to create our People table.
createPeopleTableSQL :: Query
createPeopleTableSQL =
"CREATE TABLE if not exists People (name varchar(50) NOT NULL UNIQUE \
\ ,age int CHECK(age > 0) \
\ ,id integer PRIMARY KEY NOT NULL); "
Not Null Constraint
If we try to insert a record into our database where we provide a NULL value on a column that is NOT NULL:
result <- runDBAction $ execute conn "INSERT INTO People (name, age) VALUES (?, ?)" ( (Nothing, 10) :: (Maybe Text, Int) )
where result is Left $ SQLConstraintError NotNull "People.name".
The typical error you would get from sqlite-simple would be a SQLError whose error context is "SQLite3 returned ErrorConstraint while attempting to perform step: NOT NULL constraint failed: People.name".
Unique Constraint
If we try to insert a record into our database that violates a UNIQUE constraint:
_ <- runDBAction $ execute conn "INSERT INTO People (name, age) VALUES (?, ?)" ( ("Joe", 10) :: (Text, Int) )
result <- runDBAction $ execute conn "INSERT INTO People (name, age) VALUES (?, ?)" ( ("Joe", 10) :: (Text, Int) )
where result is Left $ SQLConstraintError Unique "People.name".
The typical error you would get from sqlite-simple would be a SQLError whose error context is "SQLite3 returned ErrorConstraint while attempting to perform step: UNIQUE constraint failed: People.name".
Check Constraint
If we try to insert a record that violates some condition we've specified through CHECK:
result <- runDBAction $ execute conn "INSERT INTO People (name, age) VALUES (?, ?)" ( ("Joe", 0) :: (Text, Int) )
where result is Left $ SQLConstraintError Check "People".
The typical error you would get from sqlite-simple would be a SQLError whose error context is "SQLite3 returned ErrorConstraint while attempting to perform step: CHECK constraint failed: People".
Foreign Key Constraint
Let's make a new table where we can have a foreign key.
createJobsTableSQL :: Query
createJobsTableSQL =
"CREATE TABLE if not exists Jobs (title varchar(50) \
\ ,person_id int NOT NULL \
\ ,FOREIGN KEY(person_id) REFERENCES People(id)); "
If we try to insert a record into this table without a corresponding entry in the People table:
result <- runDBAction $ execute conn "INSERT INTO Jobs (title, person_id) VALUES (?, ?)" (("coder", -1) :: (Text, Int))
where result is Left $ SQLConstraintError ForeignKey "".
The typical error you would get from sqlite-simple would be a SQLError whose error context is "SQLite3 returned ErrorConstraint while attempting to perform step: FOREIGN KEY constraint failed".