public record Config(
    string MembersFolder,
    string TemplateFolder,
    string LettersFolder,
    string SiteUrl,
    int FeatureImageWidth,
    string ContentApiKey,
    Option<string> SendGridApiKey
);   
// ...
public static K<M, string> sendGridApiKey =>
        from k in configIO.Map(t => t.SendGridApiKey)
        from _ in when(k.IsNone, error<M>((Error)"SendGrid key not set.  No emails will be sent"))
        select (string)k;

public record SampleConfig(
    // for illustration only; I'm getting my value from somewhere that can have different kinds of errors
    // so wanted to use Fin instead of Option
    Fin<string> SendGridApiKey
    // ...
);   
// ...
public static K<M, string> sendGridApiKey =>
        from k in configIO.Map(t => t.SendGridApiKey)
        from _ in when(k.IsFail, error<M>((Error)"SendGrid key not set.  No emails will be sent"))
        //                                        ^^^here I want to pull in the error from the Fin if it fails
        select (string)k;

public static class CosmosM<M, RT>
    where RT :
    Has<M, CosmosIO>
    where M :
    Monad<M>,
    Fallible<M>
{
    public static readonly K<M, CosmosIO> cosmos = RT.Ask;

    public static K<M, PartitionKey> partitionKey(List<string> partitionKeys)
    {
        return from k in cosmos.Map(a => a.PartitionKey(partitionKeys))
               from _ in when(k.IsNone, error<M>((Error)"Invalid partition key(s) provided"))
               select (PartitionKey)k;
    }

    public static K<M, QueryDefinition> queryDefinition(string query, Dictionary<string, DbQueryParameter> queryParameters)
    {
        return from t in  cosmos.Map(a => a.QueryDefinition(query, queryParameters))
               select t;
        //               ^ problems converting a K<M, Fin<T>> to a T
    }
}