24 days of Hackage, 2015: day 16: safe; what is safety anyway?

Table of contents for the whole series

A table of contents is at the top of the article for day 1.

Day 16

(Reddit discussion)

Today I’m doing something strange: up till now, I’ve discussed libraries and tools I actually use. Today I’m discussing a library I do not use, while thinking about why I not, and why you or I might want to use it. This library is safe, which aims to protect against the unfortunate “unsafety” of common functions in the standard Prelude.

That’s a good thing, right? After all, on day 7, I promoted the use of the NonEmpty list, and again used in day 14. I like safety, but what does “safety” mean anyway?

Safety is relative

A notion of safety is always relative to some criterion, some expectation, and even more generally, in the context of some way of life. And ways of life can be safeguarded through different mechanisms, from strict protection to “style guides” to implicit social contracts and ostracism.

In the context of the safe package, the kind of safety we are concerned about is not ever wanting to see a running program crash with an exception from pure code like

*** Exception: Prelude.head: empty list

This happens if you call head on an empty list:

-- | Crashes if nobody in line.
unsafeReportFirstInLine :: [Int] -> String
unsafeReportFirstInLine nums =
  "next up: customer " ++ show (head nums)
> unsafeReportFirstInLine []
"next up: customer *** Exception: Prelude.head: empty list

The “safe” solution is don’t ever call head on a list that might be empty. There are different ways to achieve this.

Solving a human psychology problem?

One possible solution is to change the type of head.

Providing functions such as head in the Haskell Prelude is arguably a historical legacy mistake from 1990, because it encourages programmers (especially those starting out with Haskell, especially when using instructional materials that use head!) to call head. If you give people the easy ability to do something unsafe, they will surely do it, and often.

Newer language communities attack the human psychology problem by not providing an unsafe head: for example, the standard PureScript ecosystem provides a safe Data.List.head with type forall a. List a -> Maybe a but also provides an entire Data.List.Unsafe module that includes Data.List.Unsafe.head with type forall a. List a -> a.

And Elm List module provides List.head of type List a -> Maybe a.

Marking something as unsafe at least enables the writer and reader of code to make note that something might go wrong, so I think this is a good starting point for a solution. Furthermore, psychology research has shown clearly that defaults matter: if the goal is to promote safety, it is better to have the default be safe, and “opt out” explicitly to be unsafe, rather than have the default be unsafe, and “opt in” to be safe.

Unfortunately, for historical reasons, if you’re working with lists and some other data structures in Haskell, you are forced to opt in to safety, rather than opt out to unsafety.

The safe package allows you to opt in to safety.

You get functions such as Safe.headMay with type [a] -> Maybe a.

-- | Using 'Safe.headMay' and pattern matching on Maybe.
reportFirstInLine :: [Int] -> String
reportFirstInLine nums =
  case Safe.headMay nums of
    Just num -> "next up: customer " ++ show num
    Nothing -> "there are no customers in line"

Pattern-matching directly on the data structure as an alternative

In practice, I don’t use functions like headMay because I usually just pattern-match on the list itself:

-- | Using pattern matching on list.
reportFirstInLine2 :: [Int] -> String
reportFirstInLine2 [] = "there are no customers in line"
reportFirstInLine2 (num:_) = "next up: customer " ++ show num

When I think about it, though, there is something not quite right about this solution. The wildcard _ in the pattern gives away the fact that we are getting back more information than we actually need. In principle we should ask only for what we need, and headMay does precisely that. I’m basically violating conceptual encapsulation by getting back more than I need (the tail) and ignoring it.

So I believe I should really start using headMay in this kind of context, and thinking more deeply, I believe that the single reason I haven’t is that the standard Prelude didn’t provide it! It was easier to do the pattern-matching than to find safe and add it as a dependency.

How many of you think like me, and would happily use headMay if it were part of the standard Prelude, but because it is not, you use a wildcarded pattern match on a list instead?

Revisiting the headMay solution

Some of you might like using the maybe function that has type b -> (a -> b) -> Maybe a -> b, to avoid pattern matching on Maybe:

-- | No pattern matching.
reportFirstInLine3 :: [Int] -> String
reportFirstInLine3 =
  maybe "there are no customers in line" reportOne . Safe.headMay

reportOne :: Int -> String
reportOne num = "next up: customer " ++ show num

There’s also a code golf version that I do not recommend:

-- | Code golf.
reportFirstInLine4 :: [Int] -> String
reportFirstInLine4 =
  maybe "there are no customers in line"
        (("next up: customer " ++) . show)
        . Safe.headMay

Other safe goodies

Each of the ...May functions also has useful variants:

One allows specifying a default to return upon empty:

headDef :: a -> [a] -> a

Another is unsafe, but at least generates a better exception. This is useful if you know that a list is not empty, and do not want to handle the case in which it is empty, but just in case, generate an exception that if triggered, at least tells you where your “internal fatal error” came from.

headNote :: String -> [a] -> a

Exactness is a safety issue

The Safe.Exact module provides a lot of useful functions that have to do with indexing into a list or the checking the sizes of lists. Here, “safety” no longer refers to an exception. It refers to something more insidious: code you write that typechecks and runs but does the unintended thing. For example, it is very easy to use take in a way that you don’t intend because it silently allows you to “take” more elements from a list than it contains, but just assumes you know what you are doing and don’t “really” mean “take 1000 elements” but rather “take 1000 elements or if there aren’t 1000, take all the elements”. I have been bitten by take before, where I passed in an absurd number that I did not intend. So the Safe.Exact.takeExact... family of functions is quite useful. In the past, before I discovered safe, I ended up basically writing my own wrappers, and now I won’t do that again.


Finally the Safe.Foldable module is useful because Foldable is full of unsafe operations.

When you know something about your data that the type doesn’t know

A final note on exactness of list operations as a safety issue: the principled solution to inexactness when it comes to indexing into or size of lists is to turn the potential bugs into type errors, using dependent types: a “list” type that is dependent on its size. A later Day of Hackage will mention solutions.


The safe package is a nice utility library that wraps “unsafe” Prelude operations. There are technical and psychological reasons I haven’t used it, and I discussed them, but I will use it in the future when it fits my needs.

All the code

All my code for my article series are at this GitHub repo.

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