[Haskell-cafe] Code/style review

[Jan von Löwenstein]

Hi,

TL;DR
I am pretty new to Haskell and have some Haskell code I'd love to get
feedback on. (
https://github.com/SAP/bosh-cpi-haskell/tree/a586079dd9383fca030f1536116522874a086a0f
 and
https://github.com/SAP/bosh-kubernetes-cpi-release/tree/master/src/bosh-kubernetes-cpi
 )

Longer version:
I started looking into Haskell back in November last year. Pretty soon I
wanted to try something more "real-world" IO heavy in order to see how that
goes.
I came up with a side-project at work to implement a Bosh Kubernetes CPI.
Bosh (bosh.io) is a tool to install and manage distributed systems on a
range of IaaSs. In particular it is used to install Cloud Foundry (a cloud
application development platform). I guess I don't have to introduce
Kubernetes.

The interface between Bosh and its cloud provider interfaces (CPI) is a
simple command line application. Bosh provides configuration via file
provided as process call argument. It provides the method to be invoked
including arguments as JSON via stdin and expects a response via stdout.
The CPI can provide a log via stderr. (http://bosh.io/docs/cpi-api-v1.html)

The interface to Kubernetes is basically a REST api, i.e. http calls.
Currently I use a generated client based on Servant (
https://github.com/soundcloud/haskell-kubernetes)

I have put that interface into its own package.
I created two typeclasses FileSystem[1] and System[2] to abstract away the
system (I guess I should call them MonadFileSystem and MonadSystem to align
with how mtl calls the monadic classes). And I created another typeclass
MonadCpi[3] to abstract away the concrete implementation towards
potentially several infrastructures.

The Kubernetes implementation[4] is rather hacked (no tests, low
testability, etc.) and I'd like to change that. I thought about a typeclass
for REST resources (with methods create, get, list, update, delete, ...)
and I begin to have problems integrating that into my current primitives
(e.g. I'd like to have a MonadLog instance that uses my System typeclass
for printing to stderr). That's when I started to ask myself if I had
chosen the right Haskell tools and primitives for what I was doing and
thought it was time to get some feedback from more experienced Haskellers
aka you.

I know this was a pretty long text. Sorry for that. Any feedback, including
"you did it completely wrong" is appreciated - as long as it includes "have
a look here and here", "look this is what I would do", or similar.

Many thanks already for reading.

Best
Jan

[1]
https://github.com/SAP/bosh-cpi-haskell/blob/a586079dd9383fca030f1536116522874a086a0f/src/CPI/Base/System.hs#L29-L30
<https://github.com/SAP/bosh-cpi-haskell/blob/a586079dd9383fca030f1536116522874a086a0f/src/CPI/Base/System.hs#L29>
[2]
https://github.com/SAP/bosh-cpi-haskell/blob/a586079dd9383fca030f1536116522874a086a0f/src/CPI/Base/System.hs#L32-L36
[3]
https://github.com/SAP/bosh-cpi-haskell/blob/a586079dd9383fca030f1536116522874a086a0f/src/CPI/Base.hs#L126-L138

[4]
https://github.com/SAP/bosh-kubernetes-cpi-release/blob/master/src/bosh-kubernetes-cpi/src/CPI/Kubernetes.hs#L65-L296
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