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Commit 5a1e31b4 (Add infrastructure for, and two extra hlint rules)
was intended to add two *extra* hlint rules, but I didn't realise at
that time that "--hint" when first used overrides the built-in
lints. As such, since then we were basically running with just those
two rules, which resulted in many uncaught warnings/errors.

This patch fixes that (by importing the standard lint rules in our
custom hints file), and then goes to fix all the warnings that a
current hlint gives me. Compared to our current style, we have just a
few additions:

- zipWithM instead of map foo . zip …
- 'exitSuccess' instead of 'exitWith ExitSuccess'
- more uses of '.'

Additionally, we have to silence a case where hlint doesn't realise
why we are using '\e -> const (return False (e :: IOError)' instead of
just '\e -> return False' or even 'const (return False').

One warning that is generated by hlint ("Use void") can't be fixed
until we deprecate GHC 6.x, as only GHC 7 has the 'void' function in
Control.Monad.
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>

While removing split instances check from hcheck, I saw a wrong use of
'length': this should only be used when one needs the actual length of
the list, and not when one wants to check whether the list is empty or
not (in that case, 'null' should be used).

To prevent this in the future, let's add a hlint hint for it, and fix
the warning it generates in hbal.
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>

This will allow us to easily use add new lint rules that we
standardise on, if any.
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>