I am delighted to introduce a new F# library, FsConfig. FsConfig is a F# library for reading configuration data from environment variables and AppSettings with type safety

Why FsConfig?

To understand FsConfig, let’s have a look at an use case from the FsTweet application.

The FsTweet application follows The Twelve-Factor App guideline for managing the configuration data. During the application bootstrap, it retrieves its ten configuration parameters from their respective environment variables.

open System let main argv = let fsTweetConnString = Environment.GetEnvironmentVariable "FSTWEET_DB_CONN_STRING" let serverToken = Environment.GetEnvironmentVariable "FSTWEET_POSTMARK_SERVER_TOKEN" let senderEmailAddress = Environment.GetEnvironmentVariable "FSTWEET_SENDER_EMAIL_ADDRESS" let env = Environment.GetEnvironmentVariable "FSTWEET_ENVIRONMENT" let streamConfig : GetStream.Config = { ApiKey = Environment.GetEnvironmentVariable "FSTWEET_STREAM_KEY" ApiSecret = Environment.GetEnvironmentVariable "FSTWEET_STREAM_SECRET" AppId = Environment.GetEnvironmentVariable "FSTWEET_STREAM_APP_ID" } let serverKey = Environment.GetEnvironmentVariable "FSTWEET_SERVER_KEY" let port = Environment.GetEnvironmentVariable "PORT" |> uint16 // ...

Though the code snippet does the job, there are some shortcomings.

The code is verbose. There is no error handling to deal with the absence of values or wrong values. Explicit type casting

With the help of FsConfig, we can overcome these limitations by specifying the configuration data as a F# Record type.

type StreamConfig = { Key : string Secret : string AppId : string } [<Convention("FSTWEET")>] type Config = { DbConnString : string PostmarkServerToken : string SenderEmailAddress : string ServerKey : string Environment : string [<CustomName("PORT")>] Port : uint16 Stream : StreamConfig }

And then read all the associated environment variables in a single function call with type safety and error handling!

let main argv = let config = match EnvConfig.Get<Config>() with | Ok config -> config | Error error -> match error with | NotFound envVarName -> failwithf "Environment variable %s not found" envVarName | BadValue (envVarName, value) -> failwithf "Environment variable %s has invalid value" envVarName value | NotSupported msg -> failwith msg

Supported Data Types

FsConfig supports the following data types and leverages their respective TryParse function to do the type conversion.

Int16 , Int32 , Int64 , UInt16 , UInt32 , UInt64

, , , , , Byte , SByte

, Single , Double , Decimal

, , Char , String

, Bool

TimeSpan , DateTimeOffset , DateTime

, , Guid

Enum

Option Type

FsConfig allows us to specify optional configuration parameters using the option type. In the previous example, if the configuration parameter Port is optional, we can define it like

type Config = { ... - Port : uint16 + Port : uint16 option }

List Type

FsConfig also supports list type, and it expects comma separated individual values.

For example, to get mulitple ports, we can define the config as

type Config = { Port : uint16 list }

and then pass the value 8084,8085,8080 using the environment variable PORT .

Record Type

As shown in the initial example, FsConfig allows us to group similar configuration into a record type.

type AwsConfig = { AccessKeyId : string DefaultRegion : string SecretAccessKey : string } type Config = { Aws : AwsConfig }

With this configuration declaration, FsConfig read the environment variables AWS_ACCESS_KEY_ID , AWS_SECRET_ACCESS_KEY , and AWS_DEFAULT_REGION and populates the Aws field of type AwsConfig .

Environment Variable Name Convention & Customization

By default, FsConfig follows Underscores with uppercase convention, as in UPPER_CASE , for deriving the environment variable name.

type Config = { ServerKey : string }

Using this configuration declaration, FsConfig read the environment variable SERVER_KEY and populates the ServerKey field

To specify a custom prefix in the environment variables, we can make use of the Convention attribute.

[<Convention("FSTWEET")>] type Config = { ServerKey : string }

For this configuration declaration, FsConfig read the environment variable FSTWEET_SERVER_KEY and populates the ServerKey field.

We can also override the separator character _ using the Convention attribute’s optional field Separator

[<Convention("FSTWEET", Separator="-")>] type Config = { ServerKey : string }

In this case, FsConfig derives the environment variable name as FSTWEET-SERVER-KEY .

If an environment variable name is not following a convention, we can override the environment variable name at the field level using the CustomName attribute.

type Config = { [<CustomName("MY_SERVER_KEY")>] ServerKey : string }

Here, FsConfig uses the environment variable name MY_SERVER_KEY to get the ServerKey.

We can also merely customise (or control) the environment variable name generation by passing an higher-order function while calling the Get function

open FsConfig // Prefix -> string -> string let lowerCaseConfigNameCanonicalizer (Prefix prefix) (name : string) = let lowerCaseName = name.ToLowerInvariant() if String.IsNullOrEmpty prefix then name.ToLowerInvariant() else sprintf "%s-%s" (prefix.ToLowerInvariant()) lowerCaseName [<Convention("FSTWEET")>] type Config = { ServerKey : string } let main argv = let config = match EnvConfig.Get<Config> lowerCaseConfigNameCanonicalizer with | Ok config -> config | Error error -> failwithf "Error : %A" error

FsConfig computes the environment variable name as fstweet-server-key in this scenario.

Getting Individual Environment Variables

FsConfig also supports reading value directly by explicitly specifying the environment variable name

EnvConfig.Get<decimal> "MY_APP_INITIAL_BALANCE" // Result<decimal, ConfigParseError>

appSettings

Are you using appSettings in (either web.config or App.config ) to manage your configuration settings? FsConfig supports that too!

We can read the appSettings values using the AppConfig type instead of EnvConfig type.

FsConfig uses the exact name of the field to derive the appSettings key name and doesn’t use any separator by default.

type AwsConfig = { AccessKeyId : string DefaultRegion : string SecretAccessKey : string } type Config = { Port : uint16 Aws : AwsConfig } let main argv = let config = match AppConfig.Get<Config>() with | Ok config -> config | Error error -> failwithf "Error : %A" error

The above code snippet looks for appSettings values with the name Port , AwsAccessKeyId , AwsDefaultRegion , AwsSecretAccessKey and populates the respective fields.

All the customisation that we have seen for EnvConfig is applicable for AppConfig as well.

How FsConfig Works

If you are curious to know how FsConfig works and its internals then you might be interested in my blog post, Generic Programming Made Easy that deep dives into the initial implementation of FsConfig.

The current version of FsConfig is available on GitHub

Feedback

We all need people who will give us feedback. That’s how we improve - Bill Gates.

Your suggestions/feedback are welcome!

Acknowledgements

The idea of FsConfig is inspired by Kelsey Hightower’s golang library envconfig.

FsConfig uses Eirik Tsarpalis’s TypeShape library for generic programming.