example-handlers-haskell

This repository explores how to implement web handlers that perform common web application operations (logging, database calls, HTTP calls, spawning threads, catching exceptions, acquiring and releasing resources, etc.) in a few popular Haskell web frameworks (Scotty, Yesod, Servant).

It also looks at how to integrate each web framework with a custom monad stack, such as the one used by the ReaderT design pattern.

Setup

Prerequisites:

GHCup
- Used to install ghc and cabal
PostgreSQL
- Need postgresql service running and psql in path
- Also install header files to compile Haskell PostgreSQL library (libpq-dev on Ubuntu)

Create the project's example_haskell database by running:

./scripts/create-db.sh

(If you need to reset your database, run ./scripts/drop-db.sh and create it again.)

Manually source the project's environment, or install direnv to do it automatically for you:

source .envrc

This will set the proper ghc and cabal versions using GHCup, as well as the database connection string environment variable.

Build the project with:

cabal build

Running

In a first terminal, run the fake "external" service with:

cabal run example-external

In a second terminal, run one of the web framework examples:

cabal run example-scotty
# or
cabal run example-yesod
# or
cabal run example-servant

Testing

Manually test the single endpoint using a valid Cart ID that exists in the database:

$ curl -i -X POST http://localhost:3000/cart/abc123/purchase
HTTP/1.1 200 OK

{"booking_id":"TKCY693D5ACB","cart_id":"abc123","payment_id":"zTNBbSdy3vdOSnRT3xzFHviB","purchase_delay":1000}

Use a Cart ID that has already been purchased to test the error response:

$ curl -i -X POST http://localhost:3000/cart/def456/purchase
HTTP/1.1 409 Conflict

{"error":"Cart already purchased"}

Use a hard-coded Cart ID that will trigger an error from the external service:

$ curl -i -X POST http://localhost:3000/cart/ghi789/purchase
HTTP/1.1 500 Internal Server Error

{"error":"Cart purchase failed: Payment failed: Invalid card number"}

Use a Cart ID that does not exist:

$ curl -i -X POST http://localhost:3000/cart/xyz000/purchase
HTTP/1.1 404 Not Found

{"error":"Cart does not exist"}

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