FsConfig is a F# library for reading configuration data from environment variables and AppSettings with type safety

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.

1. The code is verbose.
2. There is no error handling to deal with the absence of values or wrong values.
3. 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 %s" envVarName value
      | NotSupported msg -> 
        failwith msg

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
• list of all the above types
• option of all the above types

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
}

FsConfig supports Discriminated Union Types that has cases alone.

type Color =
| Red
| Green
| Blue 

type Config = {
  ConsoleColor : Color
}

With this configuration declaration, FsConfig read the environment variable CONSOLE_COLOR and populates the ConsoleColor field of type Color.

List of Discriminated Union Types also supported!

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.

The default separator for the list can be changed if needed using the ListSeparator attribute.

  [<Convention("MYENV")>]
  type CustomListSeparatorSampleConfig = {
    ProcessNames : string list
    [<ListSeparator(';')>]
    ProcessIds : uint16 list
    [<ListSeparator('|')>]
    PipedFlow : int list    
  }

With this configuration declaration, FSConfig would be able to read the following entries from App.settings.

  <add key="MYENVProcessNames" value="conhost.exe,gitter.exe"/>
  <add key="MYENVProcessIds" value="4700;15680"/>
  <add key="MYENVPipedFlow" value="4700|15680|-1" />

A definition similar to the one shown below will allow parsing of standalone lists.

  type IntListUsingSemiColonsConfig = {
    [<ListSeparator(';')>]
    IntListUp : int list
  }

E.g. an environment containing

INT_LIST_UP=42;43;44 

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.

If you'd like to use a default value in the absence of a field value, you can make use of the DefaultValue attribute.

type Config = {
  [<DefaultValue("8080")>]
  HttpServerPort : int16
  [<DefaultValue("Server=localhost;Port=5432;Database=FsTweet;User Id=postgres;Password=test;")>]
  DbConnectionString: string
}

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.

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

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

FsConfig supports App Config for both DotNet Core and Non DotNet Core Applications.

• DotNet Core Applications (Supported from V2.0.0 or above)

• Non DotNet Core Applications (Only Supported in V0.0.6 or below)

FsConfig abstracts the configuration provider by depending on IConfigurationRoot.

let configurationRoot : IConfigurationRoot = // ...
let appConfig = new AppConfig(configurationRoot)

After creating an instance appConfig (of type AppConfig from FsConfig), you can use it to read the configuration values as below

// Reading Primitive
let result = 
  appConfig.Get<int> "processId" // Result<int, ConfigParseError>

// A Sample Record
type SampleConfig = {
  ProcessId : int
  ProcessName : string
}

// Reading a Record type
let result = 
  appConfig.Get<SampleConfig> () // Result<SampleConfig, ConfigParseError>

// A Sample Nested Record
type AwsConfig = {
  AccessKeyId : string
  DefaultRegion : string
  SecretAccessKey : string
}

type Config = {
  MagicNumber : int
  Aws : AwsConfig
}

// Reading a Nested Record type
let result = 
  appConfig.Get<Config> () // Result<Config, ConfigParseError>

Refer below for creating configurationRoot based on the file type and using FsConfig to read the values.

{
  "processId" : "123",
  "processName" : "FsConfig",
  "magicNumber" : 42,
  "aws" : {
    "accessKeyId" : "Id-123",
    "defaultRegion" : "us-east-1",
    "secretAccessKey" : "secret123"
  },
  "colors" : "Red,Green"
}

This JSON file can be read using

// Requires NuGet package
// Microsoft.Extensions.Configuration.Json
let configurationRoot =  
  ConfigurationBuilder().SetBasePath(Directory.GetCurrentDirectory())
    .AddJsonFile("settings.json").Build()

let appConfig = new AppConfig(configurationRoot)
let result = 
  appConfig.Get<Config> () // Result<Config, ConfigParseError>

<Settings>
  <ProcessId>123</ProcessId>
  <ProcessName>FsConfig</ProcessName>
  <MagicNumber>42</MagicNumber>
  <Aws>
    <AccessKeyId>Id-123</AccessKeyId>
    <DefaultRegion>us-east-1</DefaultRegion>
    <SecretAccessKey>secret123</SecretAccessKey>
  </Aws>
  <Colors>Red,Green</Colors>
</Settings>

This XML file can be read using

// Requires NuGet package
// Microsoft.Extensions.Configuration.Xml
let configurationRoot =  
  ConfigurationBuilder().SetBasePath(Directory.GetCurrentDirectory())
    .AddXmlFile("settings.xml").Build()

let appConfig = new AppConfig(configurationRoot)
let result = 
  appConfig.Get<Config> () // Result<Config, ConfigParseError>

ProcessId=123
ProcessName=FsConfig
MagicNumber=42
Colors=Red,Green

[Aws]
AccessKeyId=Id-123
DefaultRegion=us-east-1
SecretAccessKey=secret123

This INI file can be read using

// Requires NuGet package
// Microsoft.Extensions.Configuration.Ini
let configurationRoot =  
  ConfigurationBuilder().SetBasePath(Directory.GetCurrentDirectory())
    .AddIniFile("settings.ini").Build()

let appConfig = new AppConfig(configurationRoot)
let result = 
  appConfig.Get<Config> () // Result<Config, ConfigParseError>

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.

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.

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

Your suggestions/feedback are welcome!

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

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

• @tamizhvendan