Can't create "all_" query about beam

[tathougies]

The type variable is ambiguous. The compiler wants it due to the monomorphism restriction. You need to either explicitly type the query with a concrete SELECT syntax to use or declare the query polymorphic.

If you want the compiler to infer the type, you have to use the query with a particular backend somewher in the program.

Here's one way to explicitly type it polymorphically, using type holes:

query :: forall select s. Q select _ _ s _

You'll likely get constraints missing on the select type variable. This is expected. In order to run the query, the backend SELECT syntax will need to meet certain requirements, and these constraints will make sure of that.

Or monomorphically, you can do

import Database.Beam.Postgres

query :: Q PgSelectSyntax _ _ s _

Does that make sense?

[canonic-epicure]

Hm.. Theoretically it perfectly does. However, for me just trying to get my "hands dirty" and dump some sql, its a bit overwhelming. As I understand, to be able to treat the query as something "materialized" it has to belong to some backend.

With the purpose of just dumping the sql, how do I declare such query?

[tathougies]

The generated query is going to depend on the backend in use. Moreover, for certain backends -- such as postgres -- the query generated depends on the connection itself, due to connection-specific string escaping behavior. Thus, there's no canonical representation of a SQL statement.

The tutorial shows several strategies for dumping SQL. Firstly, you can dump the statements beam is about to run using withDatabaseDebug and passing in an appropriate logging function.

As the manual describes, you can also use an internal debug representation of the query to dump out a SQL-92 like expression. This query should not be used to execute the statement, due to backend-specific-escaping concerns.

To quote

dumpSqlSelect (all_ (persons exampleDb))

The backend syntax in use here is SqlSyntaxBuilder which is an idealized version of a mostly SQL92-compliant backend.

[canonic-epicure]

After changing to

{-# LANGUAGE TypeOperators, OverloadedStrings, PartialTypeSignatures #-}

module Main where

import Schema
import Database.Beam

import Database.Beam.Postgres

import Lib


main :: IO ()
main = dumpSqlSelect (all_ (_table2 db))

there's still plenty of type failures..

[canonic-epicure]

Can you perhaps check the attached archive with sample project? Its stack-based, and supposed to be unpacked inside the top beam directory, near the "beam-core", "beam-migrate" folders (which are on the latest of next-gen-beam-migrate branch). I must be missing something obvious probably.

[tathougies]

The first problem is:

Found hole:
        _table2
          :: DatabaseSettings be0 Schema.Db
             -> DatabaseEntity be db0 (TableEntity table0)

The Database constructor isn't exported from Schema.hs. This is a bug in the code generation. You can go ahead and modify the file directly, and I will upload a patch later today.

So, to be clear, change

module Schema (db, migration) where

to

module Schema (Db(..), db, migration) where

The second problem is that the beam-migrate utility generates a migratable database, which is a bit more complicated than what SqlSyntaxBuilder allows. I'll work on this today, and upload a patch this evening. Sorry about that.

In the meantime, you can try using the withDatabaseDebug technique to execute against a database.

[canonic-epicure]

Thanks for the help! I'll continue tomorrow.

[tathougies]

Try now... I think I got it working on my laptop

$ stack exec -- simple-exe
SELECT `t0`.`id` AS "res0", `t0`.`fkColumn` AS "res1" FROM "table2" AS "t0"

[canonic-epicure]

Hm.. Same branch? Doesn't work for me:

[2 of 2] Compiling Main             ( app/Main.hs, .stack-work/dist/x86_64-linux/Cabal-1.24.2.0/build/simple-exe/simple-exe-tmp/Main.o )
    
    /home/nickolay/workspace/Haskell/simple/app/Main.hs:14:37: error:
        • Couldn't match type ‘Database.Beam.Migrate.SQL.SQL92.Sql92AlterTableColumnSchemaSyntax
                                 (Database.Beam.Migrate.SQL.SQL92.Sql92AlterTableAlterTableActionSyntax
                                    (Database.Beam.Migrate.SQL.SQL92.Sql92DdlCommandAlterTableSyntax
                                       syntax0))’
                         with ‘Database.Beam.Migrate.SQL.SQL92.Sql92CreateTableColumnSchemaSyntax
                                 (Database.Beam.Migrate.SQL.SQL92.Sql92DdlCommandCreateTableSyntax
                                    syntax0)’
            arising from a use of ‘db’
          The type variable ‘syntax0’ is ambiguous
        • In the first argument of ‘_table2’, namely ‘db’
          In the first argument of ‘all_’, namely ‘(_table2 db)’
          In the first argument of ‘dumpSqlSelect’, namely
            ‘(all_ (_table2 db))’

[tathougies]

You pulled the latest beam-core? Sometimes you have to do stack clean beam-core to have it rebuild

[canonic-epicure]

Yes, works after "stack clean"! Yay :) Thanks for the help, I'll be figuring the next step..

[tathougies]

@samuraijack Wonderful! Can I close this issue, then?

[canonic-epicure]

Yes, sure.