How to use the value from one validation in another validation

[speterson-zoll]

We have an extension to IConfiguration that lets us reference AWS SSM parameters like so:

configuration[MyKeyName]

I need to check for one parameter name, and, if not empty, use that value as part of other keys to look up other parameters. For example, in imperative code I might have:

var env = configuration[MyKeyName];
var dbHostName = configuration[$"{DbHostNameKey}-{env}"];
var dbName = configuration[$"{DbNameKey}-{env}"];
var port = configuration[$"{DbPortKey}-{env}"];
var user = configuration[$"{DbUserKey}"];
var pass = configuration[$"{DbPasswordKey}"];

Notice that the env value is used as part of the key name for some of the other look-ups.

I would like to approach this more functionally, such that I get a validation error if the value from configuration[MyKeyName] is null or empty, and then for each value that's dependent on the env value in the key. I'm afraid I have my head in a knot between Apply, Bind and Match. I have used Apply to validate and create an object as per the examples, but the examples don't use the value in the Validation<Error, string> object in any subsequent validations, which is what I need to do here. I also need to be able to get each validated value into an object like so:

private sealed class DbAuthInfo
  {
    public string DbHost { get; set; }
    public string DbName { get; set; }
    public uint DbPort { get; set; }
    public string DbUser { get; set; }
    public string DbPass { get; set; }
  }

Thanks for your help!

[louthy]

Just put them in a LINQ expression:

Validation<Error, string> GetConfig(string key) => …

var res = from v in GetConfig(“key1”)
          from v2 in GetConfig(v)
          select v2;

[speterson-zoll]

That did it, thank you! I'm still getting used to functional ways of doing things and I needed a push. What I came up with:

private static Validation<Error, string> GetConfig(IConfiguration configuration, string key, string parameterName) =>
    Optional(configuration).Match(
      Some: config => ValidateStringNotEmpty(config[key], parameterName),
      None: () => Fail<Error, string>(Error.New($"The {nameof(configuration)} object cannot be null")));

which is then used by an IServiceCollection extension method:

public static void RegisterDbContexts(this IServiceCollection services, IConfiguration configuration)
  {
    var dbHost =
      from env in GetConfig(configuration, ConfigurationKeys.Environment.EnvKey, nameof(ConfigurationKeys.Environment.EnvKey))
      from dbh in GetConfig(configuration, $"{Ssm.DbHost}-{env}", $"{nameof(Ssm.DbHost)}-{env}")
      select dbh;

    var dbName =
      from env in GetConfig(configuration, ConfigurationKeys.Environment.EnvKey, nameof(ConfigurationKeys.Environment.EnvKey))
      from dbn in GetConfig(configuration, $"{Ssm.DbName}-{env}", $"{nameof(Ssm.DbName)}-{env}")
      select dbn;

    var dbPort =
      from env in GetConfig(configuration, ConfigurationKeys.Environment.EnvKey, nameof(ConfigurationKeys.Environment.EnvKey))
      from dbp in GetConfig(configuration, $"{Ssm.DbPort}-{env}", $"{nameof(Ssm.DbPort)}-{env}")
      from parsedPort in parseUInt(dbp).ToValidation(Error.New($"Cannot parse {dbp} to an integer"))
      select parsedPort;

    var dbUser = from adu in GetConfig(configuration, $"{Ssm.DbUser}", nameof(Ssm.DbUser))
      select adu;

    var dbPass = from adp in GetConfig(configuration, $"{Ssm.DbPassword}", nameof(Ssm.DbPassword))
      select adp;

    var dbConnectInfo = (dbHost, dbName, dbPort, dbUser, dbPass)
      .Apply((dh, dn, dp, du, ds) => new MySqlConnectionStringBuilder
      {
        Server = dh,
        Database = dn,
        Port = dp,
        UserID = du,
        Password = ds,
        AllowZeroDateTime = false
      });

    dbConnectInfo.Match(
      Succ: builder => services.AddSingleton<IOurCompanyConnection, OurCompanyConnection>(connection =>
        new OurCompanyConnection(() => new MySqlConnection(builder.ConnectionString))),
      Fail: seq => raise<Exception>(new Exception(seq.ToFullString())));
  }

It works well in that I get a parameter specific message if one or more parameters is null or empty.