If you do nothing else, avoid use of partial functions from the Prelude! import Utility.PartialPrelude helps avoid this by defining conflicting functions for all the common ones. Also avoid !!, it's partial too.

The rest of this coding style is followed to keep the code in Propellor consistent. You don't have to follow these rules in your own config.hs, or in Propellor modules that you don't intend to get merged into mainstrain Propellor.

Start a module with a comment indicating what software it provides properties for, and who maintains the module.

-- | Maintainer: Your Name Here <optional-email-address@example.org>
-- Support for the Foo daemon <https://foo.example.com/>

module Propellor.Property.Foo

Use tabs for indentation.

Code should make sense with any tab stop setting, but 8 space tabs are the default. With 8 space tabs, code should not exceed 80 characters per line. (With larger tabs, it may of course.)

Use spaces for layout. For example, here spaces (indicated with .) are used after the initial tab to make the third test line up with the others.

    when (foo_test || bar_test ||
        print "hi"

As a special Haskell-specific rule, "where" clauses are indented with two spaces, rather than a tab. This makes them stand out from the main body of the function, and avoids excessive indentation of the where cause content. The definitions within the where clause should be put on separate lines, each indented with a tab.

main = do
    foo = ...
    bar = ...

Where clauses for instance definitions and modules tend to appear at the end of a line, rather than on a separate line.

module Foo (Foo, mkFoo, unFoo) where
instance Show Property where

When a function's type signature needs to be wrapped to another line, it's typical to switch to displaying one parameter per line.

foo :: Bar -> Baz -> (Bar -> Baz) -> IO Baz

    :: Bar
    -> Baz
    -> (Bar -> Baz)
    -> IO Baz

Note that the "::" then starts its own line. It is not put on the same line as the function name because then it would not be guaranteed to line up with the "->" at all tab width settings. Similarly, guards are put on their own lines:

splat i
    | odd i = error "splat!"
    | otherwise = i

Multiline lists and record syntax are written with leading commas, that line up with the open and close punctuation.

list =
    [ item1
    , item2
    , item3

foo = DataStructure
    { name = "bar"
    , address = "baz"

Similarly, data structures line up the leading = with the following |

data Foo
    = Bar
    | Baz
    | Quux Foo
    deriving (Eq, Ord)

Module imports are separated into two blocks, one for third-party modules, and one for modules that are part of propellor. (Additional blocks can be used if it makes sense.)

Using tabs for indentation makes use of let .. in particularly tricky. There's no really good way to bind multiple names in a let clause with tab indentation. Instead, a where clause is typically used. To bind a single name in a let clause, this is sometimes used:

foo = let x = 42
    in x + (x-1) + x

If you feel that this coding style leads to excessive amounts of horizontal or vertical whitespace around your code, making it hard to fit enough of it on the screen, consider finding a better abstraction, so the code that does fit on the screen is easily understandable. ;)

Note for emacs users: You can put the following snippet into a file called .dir-locals.el at root of propellor's source tree to use tabs for indentation:

((nil . ((indent-tabs-mode . t)
         (tab-width . 8)
         (fill-column . 80)))
 ;; Warn about spaces used for indentation:
 (haskell-mode . ((eval . (highlight-regexp "^ +")))))

Also consider haskell-tab-indent-mode. The standard indentation modes that come with haskell-mode do not work well with tabs for indentation. This mode works well for hacking on Propellor.