What is the difference between Pattern Matching and Guards?

Question

I am very new to Haskell and to functional programming in general. My question is pretty basic. What is the difference between Pattern Matching and Guards?

Function using pattern matching

check :: [a] -> String
check [] = "Empty"
check (x:xs) = "Contains Elements"

Function using guards

check_ :: [a] -> String
check_ lst
    | length lst < 1 = "Empty"
    | otherwise = "Contains elements"

To me it looks like Pattern Matching and Guards are fundamentally the same. Both evaluate a condition, and if true will execute the expression hooked to it. Am I correct in my understanding?

In this example I can either use pattern matching or guards to arrive at the same result. But something tells me I am missing out on something important here. Can we always replace one with the other?

Could someone give examples where pattern matching is preferred over guards and vice versa?

Answer 1

Actually, they're fundamentally quite different! At least in Haskell, at any rate.

Guards are both simpler and more flexible: They're essentially just special syntax that translates to a series of if/then expressions. You can put arbitrary boolean expressions in the guards, but they don't do anything you couldn't do with a regular if.

Pattern matches do several additional things: They're the only way to deconstruct data, and they bind identifiers within their scope. In the same sense that guards are equivalent to if expressions, pattern matching is equivalent to case expressions. Declarations (either at the top level, or in something like a let expression) are also a form of pattern match, with "normal" definitions being matches with the trivial pattern, a single identifier.

Pattern matches also tend to be the main way stuff actually happens in Haskell--attempting to deconstruct data in a pattern is one of the few things that forces evaluation.

By the way, you can actually do pattern matching in top-level declarations:

square = (^2)

(one:four:nine:_) = map square [1..]

This is occasionally useful for a group of related definitions.

GHC also provides the ViewPatterns extension which sort of combines both; you can use arbitrary functions in a binding context and then pattern match on the result. This is still just syntactic sugar for the usual stuff, of course.

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As for the day-to-day issue of which to use where, here's some rough guides:

• Definitely use pattern matching for anything that can be matched directly one or two constructors deep, where you don't really care about the compound data as a whole, but do care about most of the structure. The @ syntax lets you bind the overall structure to a variable while also pattern matching on it, but doing too much of that in one pattern can get ugly and unreadable quickly.

• Definitely use guards when you need to make a choice based on some property that doesn't correspond neatly to a pattern, e.g. comparing two Int values to see which is larger.

• If you need only a couple pieces of data from deep inside a large structure, particularly if you also need to use the structure as a whole, guards and accessor functions are usually more readable than some monstrous pattern full of @ and _.

• If you need to do the same thing for values represented by different patterns, but with a convenient predicate to classify them, using a single generic pattern with a guard is usually more readable. Note that if a set of guards is non-exhaustive, anything that fails all the guards will drop down to the next pattern (if any). So you can combine a general pattern with some filter to catch exceptional cases, then do pattern matching on everything else to get details you care about.

• Definitely don't use guards for things that could be trivially checked with a pattern. Checking for empty lists is the classic example, use a pattern match for that.

• In general, when in doubt, just stick with pattern matching by default, it's usually nicer. If a pattern starts getting really ugly or convoluted, then stop to consider how else you could write it. Besides using guards, other options include extracting subexpressions as separate functions or putting case expressions inside the function body in order to push some of the pattern matching down onto them and out of the main definition.

Answer 2

To me it looks like Pattern Matching and Guards are fundamentally the same. Both evaluate a condition, and if true will execute the expression hooked to it. Am I correct in my understanding?

Not quite. First pattern matching can not evaluate arbitrary conditions. It can only check whether a value was created using a given constructor.

Second pattern matching can bind variables. So while the pattern [] might be equivalent to the guard null lst (not using length because that'd not be equivalent - more on that later), the pattern x:xs most certainly is not equivalent to the guard not (null lst) because the pattern binds the variables x and xs, which the guard does not.

A note on using length: Using length to check whether a list is empty is very bad practice, because, to calculate the length it needs to go through the whole list, which will take O(n) time, while just checking whether the list is empty takes O(1) time with null or pattern matching. Further using `length´ just plain does not work on infinite lists.

Answer 3

For one, you can put boolean expressions within a guard.

For example:

Just as with list comprehensions, boolean expressions can be freely mixed with among the pattern guards. For example:

f x | [y] <- x
    , y > 3
    , Just z <- h y
    = ...

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Update

There is a nice quote from Learn You a Haskell about the difference:

Whereas patterns are a way of making sure a value conforms to some form and deconstructing it, guards are a way of testing whether some property of a value (or several of them) are true or false. That sounds a lot like an if statement and it's very similar. The thing is that guards are a lot more readable when you have several conditions and they play really nicely with patterns.

Answer 4

In addition to the other good answers, I'll try to be specific about guards: Guards are just syntactic sugar. If you think about it, you will often have the following structure in your programs:

f y = ...
f x =
  if p(x) then A else B

That is, if a pattern matches, it is followed right after by a if-then-else discrimination. A guard folds this discrimination into the pattern match directly:

f y = ...
f x | p(x) = A
    | otherwise = B

(otherwise is defined to be True in the standard library). It is more convenient than an if-then-else chain and sometimes it also makes the code much simpler variant-wise so it is easier to write than the if-then-else construction.

In other words, it is sugar on top of another construction in a way which greatly simplifies your code in many cases. You will find that it eliminates a lot of if-then-else chains and make your code more readable.