At a certain point in the lifetime of your IHP app you will want to add your own config parameters, e.g. for managing secrets, API keys or external services. This guide explains the best practises for doing that.

Custom Configuration

Dealing with Secrets

Sometimes you want to have a custom configuration flag inside your application.

The recommended way is to declare a custom newtype in Config/Config.hs like this:

-- Config.hs

newtype StripePublicKey = StripePublicKey Text

We want our new config parameter to be filled from a STRIPE_PUBLIC_KEY env variable. Therefore we add this to our Config.hs:

module Config where

newtype StripePublicKey = StripePublicKey Text

config :: ConfigBuilder
config = do
    -- ...
    stripePublicKey <- StripePublicKey <$> env @Text "STRIPE_PUBLIC_KEY"
    option stripePublicKey

Now the app reads the STRIPE_PUBLIC_KEY env variable at startup and makes it available to the app.

Before we proceed we should add a default value for this in dev mode. Open the start script and add the following env variables:

# Add this before the `RunDevServer` call at the end of the file
export STRIPE_PUBLIC_KEY="pk_test_..."

# Finally start the dev server

Using Custom Config Parameters

You can now access the StripePublicKey parameter by calling getAppConfig @Config.StripePublicKey:

import qualified Config

action MyAction = do
    let (StripePublicKey stripePublicKey) = getAppConfig @Config.StripePublicKey

    putStrLn ("Stripe public key: " <> stripePublicKey)

Environment Variables

Reading Environment Variables

Inside Config/Config.hs you can use env to read environment variables.

module Config where

config :: ConfigBuilder
config = do
    someString <- env @Text "SOME_STRING"

The env function will raise an error if the env var is not defined.

The env function can also deal with other common types:

module Config where

config :: ConfigBuilder
config = do
    maxRetryCount <- env @Int "MAX_RETRY_COUNT"
    byteString <- env @ByteString "SOME_BYTESTRING"

Default Values

Use envOrDefault to provide a default value for an env var:

module Config where

config :: ConfigBuilder
config = do
    redisPort <- envOrDefault @Int 6379 "REDIS_PORT"

Optional Env Variables

When an env variable is optional and has no good default value, use envOrNothing. It will return Nothing if the env variable is not set:

module Config where

config :: ConfigBuilder
config = do
    redisUrl :: Maybe Text <- envOrNothing "REDIS_URL"

Custom Parser

When you’re dealing with a custom enum type it can be useful to write a custom env parser by implementing an EnvVarReader:

module Config where

config :: ConfigBuilder
config = do
    ipAddrSource :: IPAddrSource <- envOrDefault "IP_ADDR_SOURCE" FromSocket

data IPAddrSource = FromSocket | FromHeader

instance EnvVarReader RequestLogger.IPAddrSource where
    envStringToValue "FromHeader" = Right RequestLogger.FromHeader
    envStringToValue "FromSocket" = Right RequestLogger.FromSocket
    envStringToValue otherwise    = Left "Expected 'FromHeader' or 'FromSocket'"

Custom Middleware

IHP provides an “escape-hatch” from the framework with the CustomMiddleware option. This can be used to run any WAI middleware after IHP’s middleware stack, allowing for possibilities such as embedding a Servant or Yesod app into an IHP app, adding GZIP compression, or any other number of possibilities. See wai-extra for examples of WAI middleware that could be added.

The following example sets up a custom middleware that infers the real IP using X-Forwarded-For and adds a custom header for every request.

module Config where

import Network.Wai.Middleware.AddHeaders (addHeaders)
import Network.Wai.Middleware.RealIp (realIp)

config :: ConfigBuilder
config = do
    option $ CustomMiddleware $ addHeaders [("X-My-Header", "Custom WAI Middleware!")] . realIp

Compression Middleware

We can compress assets using gzip or brotli. First, let’s add the required Haskell dependencies: In your default.nix file, add:

        haskellDeps = p: with p; [

            # Wai Middleware
            wai-middleware-brotli # <-- Add This Dependency
            wai-extra # <-- And This One

Run, nix-shell --run 'make -B .envrc' to update the environment. Once that succeeds, we can use it in your Config/Config.hs:

Add two imports, one for Gzip compression, another for Brotli compression:

module Config where
import Network.Wai.Middleware.Brotli -- <-- Add This Import
import Network.Wai.Middleware.Gzip -- <-- And This One

And then create a function compressionMiddleware that combines (composes) Gzip and Brotli compression middleware’s into one middleware:

-- | Gzip And Brotli Compression Middleware
compressionMiddleware :: CustomMiddleware
compressionMiddleware =
        -- With `GzipCompress` and `BrotliCompress` options, it will compress per request.
        gzipSettings = def { gzipFiles = GzipCompress }
        brotliSettings = defaultSettings { brotliFilesBehavior = BrotliCompress }
        CustomMiddleware (gzip gzipSettings . brotli brotliSettings)

Lastly, we can use it as:

config :: ConfigBuilder
config = do
    option compressionMiddleware -- <-- Here we add our middleware

The default behavior for GzipCompress and BrotliCompress is to compress files on the fly. You can customize this behavior, take a look at the brotli config and gzip config.

Also notice CustomMiddleware (gzip gzipSettings . brotli brotliSettings), It’s important that brotli middleware wraps the gzip middleware, so the responses are not compressed by both, if the client supports brotli, compress with brotli, otherwise gzip, fallback to no compression.

By default all text/* content types will be compressed, including application/json, application/javascript, application/ecmascript and image/x-icon. Simply put, html, text, css, javascript, json and icons.