Documentation

A type that can be converted to a single CSV record.

An example type and instance:

data Person = Person { name :: !Text, age :: !Int }

instance ToNamedRecord Person where
    toNamedRecord (Person name age) = namedRecord [
        "name" .= name, "age" .= age]

A type that has a default field order when converted to CSV. This class lets you specify how to get the headers to use for a record type that's an instance of ToNamedRecord.

To derive an instance, the type is required to only have one constructor and that constructor must have named fields (also known as selectors) for all fields.

Right: data Foo = Foo { foo :: !Int }

Wrong: data Bar = Bar Int

If you try to derive an instance using GHC generics and your type doesn't have named fields, you will get an error along the lines of:

<interactive>:9:10:
    No instance for (DefaultOrdered (M1 S NoSelector (K1 R Char) ()))
      arising from a use of ‘Data.Csv.Conversion.$gdmheader’
    In the expression: Data.Csv.Conversion.$gdmheader
    In an equation for ‘header’:
        header = Data.Csv.Conversion.$gdmheader
    In the instance declaration for ‘DefaultOrdered Foo’