Represents an SQL data type that maps to an OCaml type 'a.

custom ~ty ~repr creates a new SQL type that is represented by the Caqti type ty, and is represented in a SQL query as repr.

For example, you might define the BOOL datatype as follows:

  let bool = Type.custom ~ty:Caqti_type.bool ~repr:"BOOLEAN"

Note Petrol doesn't implement the boolean type using this function, and uses a slightly more efficient internal encoding, but for more bespoke custom user types this function should be sufficient.

pp_value ty fmt vl returns a pretty printer for values of a type ty.

Poor man's type classes for numeric arguments.