Pattern Matching in Haskell

Pattern matching is a feature of logic programming and functional programming. It should not be confused with matching strings. We can define pattern matching in the context of purely functional languages as a dispatch mechanism that chooses which variant of a function is the correct one to call. This feature is fundamental in making programming paradigms declarative instead of imperative. Additionally, implementing a translator pattern and external polymorphism is trivial in languages with pattern matching.

Pattern matching in Haskell involves three main things:

  • Specifying patterns to which some data should conform to
  • Checking to see if indeed the data conforms to the set pattern
  • Deconstructing the data according to those patterns

We can say that pattern matching is the ability of a compiler to compare both the content and the form of two expressions. It can be used in the following two ways:

  • Assigning all or part of a data structure to several or one variables in a single expression
  • The form and content of the arguments of a function can make it have several clauses on definitions. To initialize variables that will be used in the clause, we combine the second use with the first one.

A pattern match offers the additional benefit of making an assertion about the content and form of a variable in both the uses mentioned above.

Things that you should note

  • The patterns list is ordered. This means that various functions are called in order when determining a match.
  • It is not a requirement that clauses be disjoint.
  • Pattern matching in Haskell examples often uses the match-anything wild-card as the last clause. This matches any and all arguments.
  • An error will be generated if there are no patterns that match

Functions play a critical role in Haskell. We can define separate function bodies for different patterns when defining functions. Also, we can pattern match on any data type. For example, characters, tuples, numbers, lists, etc. Doing this will lead to a very simple, expressive and readable code.

When you make a trivial function that checks if the number supplied to it is 7 or not, the patterns will be checked from top to bottom. The corresponding function will be used when the argument of the function conforms to a pattern. In this example, the only way a number can conform to the first pattern is if it is seven. It will fall through the second pattern if it is not seven. The second pattern matches anything and binds it to:

ghci  1> let {lcky : : (integral a) =>a -> string;

lucky 7 = “ The lucky number is seven!”;

lucky x = “sorry, luck is not on your side pal”}

When we are not in ghci, then the Haskell syntax for functions is much cleaner. The let keyword, semicolons or curly braces are not used. However, white spaces or block indentation is significant.

Pattern Matching Failures

Pattern matching can fail if we try to call a function with an input that we do not expect. The ghci will complain that there are exhaustive patterns. It is for this reason that we should always include catch-all patterns when making patterns. This will ensure that our program doesn’t crash if we get an unexpected input.

Lists in pattern matching

Lists can also be used in pattern matching in Haskell. It is possible to match with an empty list [ ] or any pattern that involves : and the empty list. For example pattern, X : XS binds the head of the list to x and the rest of it to XS. XS will end up being an empty list even if there is only one element.

Recursive functions are used a lot with the X : XS pattern. However, you should note that patterns that have : in them only match against lists of length 1 or more. We can use something like x :y : z : zs if we want to bind the first three elements to variables and the rest of the list to another variable.

Types of pattern matching in Haskell

  • Guarded Pattern matching

In this case, patterns will be matched only if the predicate tests are proven to be true. Guarded pattern matching is quite complicated because matches on pure structural data are prevented.

  • Structural pattern matching

This type of pattern matching does not just match on the value, but also the syntactic and structural properties of the value.

  • Partial-structural pattern matching

Structural pattern matching describes matching against complete structure. However, we can also perform matches against abstracted or partial structures with variables that exist in the structure description. This is called partial-structural pattern matching.

Others are:

  • Heuristic pattern matching
  • Ordered pattern matching

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