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Haskell on Miren

Haskell isn't auto-detected, so you deploy it with a Dockerfile.miren that compiles a binary with Cabal and runs it on a minimal image. This guide uses the Scotty web framework as the example.

Let your agent do this

Ask your AI coding agent to "set up this Haskell app on Miren" after installing the Miren agent skills. It adds the Dockerfile.miren, confirms the server binds 0.0.0.0:$PORT, wires up environment variables, and deploys — using this page as its reference.

Do you need a Dockerfile?​

Yes. Miren doesn't auto-detect Haskell, so add a Dockerfile.miren to your project root. Miren builds from it instead of guessing the stack — see Using Dockerfile.miren.

Want native support?

Miren auto-detects and builds common stacks (Python, Node, Bun, Go, Ruby, Rust) without a Dockerfile. This language isn't one of them yet — if you'd like first-class support, request it.

Bind to the injected port​

Miren injects PORT and routes traffic to it. Scotty (via Warp) binds to all interfaces already, so you just read PORT:

{-# LANGUAGE OverloadedStrings #-}
module Main where

import Web.Scotty
import System.Environment (lookupEnv)

main :: IO ()
main = do
  portStr <- lookupEnv "PORT"
  let port = maybe 8080 read portStr
  scotty port $
    get "/" $ text "Hello from Haskell on Miren!\n"

The Dockerfile​

Create Dockerfile.miren in your project root. The build uses the official haskell image (GHC + Cabal), then copies the binary onto a slim runtime:

# ----- Build stage -----
FROM haskell:9.6.6 AS builder

WORKDIR /app
COPY . .
RUN cabal update
RUN cabal build
RUN cp "$(cabal list-bin haskell-bench)" /app/app.bin

# ----- Runtime stage -----
FROM debian:12-slim

RUN apt-get update -y \
    && apt-get install -y libgmp10 zlib1g ca-certificates \
    && apt-get clean && rm -rf /var/lib/apt/lists/*

COPY --from=builder /app/app.bin /usr/local/bin/app

EXPOSE 8080
CMD ["app"]

cabal list-bin <exe> prints the built binary's path, which the build copies to a stable location. Replace haskell-bench with your executable name from the .cabal file. A minimal .cabal:

cabal-version:      2.4
name:               haskell-bench
version:            0.1.0

executable haskell-bench
    main-is:          Main.hs
    hs-source-dirs:   app
    build-depends:    base, scotty, text
    default-language: Haskell2010
    ghc-options:      -threaded -rtsopts "-with-rtsopts=-N"
Warp needs the threaded runtime

-threaded in ghc-options is required. Scotty runs on Warp, which uses GHC's event manager — without the threaded runtime the app builds and starts but crashes on the first request with getSystemTimerManager: the TimerManager requires linking against the threaded runtime. -with-rtsopts=-N lets it use all available cores.

Haskell builds are slow

GHC compiles Scotty and its dependencies from source, so the first build takes several minutes. Miren caches image layers, so rebuilds that don't change dependencies are much faster.

.dockerignore​

.git
dist-newstyle

Set up the app​

Even with a Dockerfile.miren, Miren needs at least one service defined — it doesn't use the image's CMD as the start command. Add a Procfile:

web: /usr/local/bin/app

Then create .miren/app.toml naming your app and deploy from your project root:

name = "haskell-bench"
miren deploy
Deploying without a service fails

If no service is defined, the build succeeds but the deploy stops with no services defined: please define at least one service in a Procfile or .miren/app.toml.

Environment variables​

Set variables with miren env set — -e for plain values, -s for secrets (masked in output and logs). Read them with System.Environment.lookupEnv:

miren env set -e LOG_LEVEL=info
miren env set -s DATABASE_URL

You can also declare variables in .miren/app.toml:

[[env]]
key = "DATABASE_URL"
value = ""
required = true
sensitive = true

See App Configuration — Environment Variables.

Agent quick reference​

  • Detection: none — requires Dockerfile.miren (native binary)
  • Build: cabal build on the haskell image; copy $(cabal list-bin <exe>) to a slim image
  • Threaded runtime: add ghc-options: -threaded — Warp/Scotty crashes on first request without it
  • Runtime libs: libgmp10 zlib1g on debian-slim
  • Service is required: define a Procfile (web: /usr/local/bin/app) — the image CMD is not used
  • Port: read lookupEnv "PORT"; Scotty/Warp binds all interfaces
  • Env vars: miren env set -e/-s; read with lookupEnv

Next steps​