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Monthly Downloads: 37
Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
Tags: Database     Sqlite    

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README

sqlite-simple-errors

Hackage Build Status 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".