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What is pattern matching in Haskell and how is it related to guarded equations?
I've tried looking for a simple explanation, but I haven't found one.
EDIT: Someone tagged as homework. I don't go to school anymore, I'm just learning Haskell and I'm trying to understand this concept. Pure out of interest.
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Pattern matching in computer science is the checking and locating of specific sequences of data of some pattern among raw data or a sequence of tokens. Unlike pattern recognition, the match has to be exact in the case of pattern matching.
Pattern matching is used to determine whether source files of high-level languages are syntactically correct. It is also used to find and replace a matching pattern in a text or code with another text/code. Any application that supports search functionality uses pattern matching in one way or another.
The pattern is a general description of the format of the string. It can consist of text or the special characters X, A, and N preceded by an integer used as a repeating factor. X stands for any characters, A stands for any alphabetic characters, and N stands for any numeric characters.
Pattern matching occurs when evaluating a function call, case- receive- try- expressions and match operator (=) expressions. In a pattern matching, a left-hand side pattern is matched against a right-hand side term. If the matching succeeds, any unbound variables in the pattern become bound.
In a nutshell, patterns are like defining piecewise functions in math. You can specify different function bodies for different arguments using patterns. When you call a function, the appropriate body is chosen by comparing the actual arguments with the various argument patterns. Read A Gentle Introduction to Haskell for more information.
Compare:
with the equivalent Haskell:
fib 0 = 1 fib 1 = 1 fib n | n >= 2 = fib (n-1) + fib (n-2) Note the "n ≥ 2" in the piecewise function becomes a guard in the Haskell version, but the other two conditions are simply patterns. Patterns are conditions that test values and structure, such as x:xs, (x, y, z), or Just x. In a piecewise definition, conditions based on = or ∈ relations (basically, the conditions that say something "is" something else) become patterns. Guards allow for more general conditions. We could rewrite fib to use guards:
fib n | n == 0 = 1 | n == 1 = 1 | n >= 2 = fib (n-1) + fib (n-2) If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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