Clarification of backendMigrationScript's rendering function

[peterbecich]

Question about backendMigrationScript: https://hackage.haskell.org/package/beam-migrate-0.5.3.1/docs/Database-Beam-Migrate-Simple.html#v:backendMigrationScript

-- | Given a function to convert a command to a 'String', produce a script that
-- will execute the given migration. Usually, the function you provide
-- eventually calls 'displaySyntax' to rendere the command.
backendMigrationScript :: BeamSqlBackend be
                       => (BeamSqlBackendSyntax be -> String)
                       -> Migration be a
                       -> String
backendMigrationScript render mig =
  migrateScript ((++"\n") . T.unpack) ((++"\n") . render) (migrationStep "Migration Script" (\() -> mig))

Would you provide an example of the function to convert a command to a 'String', the render argument?

---

I have a test ready to go:
https://github.com/peterbecich/beam/blob/db771f0f58a99d96ac29210fb27da114b537a488/beam-postgres/examples/app/Main.hs#L16

Given:

migration :: MigrationSteps Postgres () (CheckedDatabaseSettings Postgres Pagila.Schema.V0002.PagilaDb)
migration = migrationStep "Initial commit" V0001.migration >>>
            migrationStep "Add film actor, inventory, rental table" V0002.migration

https://github.com/peterbecich/beam/blob/db771f0f58a99d96ac29210fb27da114b537a488/beam-postgres/examples/src/Pagila/Schema.hs#L18-L20

and a modification to the backendMigrationScript function:

backendMigrationStepsScript :: BeamSqlBackend be
                            => (BeamSqlBackendSyntax be -> String)
                            -> MigrationSteps be () a
                            -> String

https://github.com/peterbecich/beam/blob/db771f0f58a99d96ac29210fb27da114b537a488/beam-migrate/Database/Beam/Migrate/Simple.hs#L342-L350

, I am hoping it will print the SQL generated from these Beam migration V0001 and V0002.

Thanks

[LaurentRDC]

Hi Peter,

The key to understanding (BeamSqlBackendSyntax be -> String) is to understand BeamSqlBackendSyntax be.

BeamSqlBackendSyntax is a type family, and also known as a type function, which means that BeamSqlBackendSyntax be is a concrete type which depends on the backend. For example, for the Postgres backend, BeamSqlBackendSyntax be = PgCommandSyntax` .

This means that you're looking for a rendered function of the type (PgCommandSyntax -> String).

Drilling deeper, you'll see that any PgCommandSyntax is composed of a PgSyntax and some other data that does not need to be printed. So, you're looking for a function of type PgSyntax -> String.

Turns out there is something very close called pgRenderSyntaxScript, which you can use to build your render function.