A Ruby Pattern Matching Library
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README.markdown

Matchmaker is a powerful and expressive DSL that brings pattern matching into Ruby. Often we need to check if an object is what we expect, and if so, we want to take it apart. Pattern matching is an expressive idiom from functional programming languages to make checking on an object and taking it apart concisely readable.

Install

> sudo gem install hayeah-matchmaker
> irb -rubygems
irb> require 'matchmaker'
irb> Case(1) { of(1) }
# => true

Case Statement

A case statement can have many alternative patterns to match. The patterns are run in sequence until the first one that matches. If no pattern matches, a Case::NoMatch exception is raised.

Case(1) {
 of(1)
 of(:b)
} # => true

Case(2) {
 of(1)
 of(:b)
} # => raise Case::NoMatch

Case(:b) {
 of(1)
 of(:b)
} # => true

Matchmaker uses the Case DSL to build a runnable pattern matcher. You can store the pattern matcher in a constant or variable, so Matchmaker doesn't have to build it everytime you use it.

c = Case.new {
  of(1)
  of(:b)
}
c.match(1)  # => true
c.match(0)  # => Case::NoMatch
c.match(:b) # => true

For each pattern in the case statement, there can be an action associated with it. The action is run when the pattern of that branch matches, and the result of the action returned:

c = Case.new {
 of(1) { :a }
 of(2) { :b }
 of(3) # { true } by default
}
c.match(1) # => :a
c.match(2) # => :b
c.match(3) # => true

Simple Patterns

Literals ruby objects are matched by ==

of(1)   # matches 1
of(:a)  # matches :a
of("a") # matches "a"

A regex is used to match string

# matches "0abc0"
of(/abc/) 

A class is used to match objects of the that class

# matches "abc"
of(String)

A range can be used to match a range of integers

# matches integer within 1 and 10
of(1..10) 

An array should be an array of patterns that matches an array object, such that each element in the array should match its pattern.

# matches ["a",[],{}]
of([String,Array,Hash]) 

Destructuring and Pattern Guard

There are patterns to match common ruby objects. For example, Case#string is a method of the DSL to build the string pattern.

of(string)        # matches any string
of(string("a"))   # matches "a"
of(string(/foo/)) # matches any string of that regexp

These pattern methods can make variable bindings of the matched object,

Case([1,"a"]) {
 of([1,string(/a/,:a)]) { a }
} # => "a"

In this example, the pattern sets the variable :a to the matched string. These variables are actually bindings. Binding of the same name in different patterns must equal to each other, otherwise the overall pattern won't match.

c = Case.new {
 of([string(/a/,:a),string(/b/,:a)])
}
c.match(["ab","ab"]) # => true
c.match(["ab","ba"]) # Case::NoMatch

A guard is an arbitray ruby block associated with a pattern, so that a pattern matches iff the guard returns true.

c = Case.new {
 of(string(/a/) {|s| s.length == 2 })
}
c.match("ab")  # => true
c.match("ba")  # => true
c.match("abc") # Case::NoMatch

Value Patterns

These patterns are used to match a single Ruby object. See later for structural patterns for ways to combine these value patterns. All these methods can make binding and take a guard, as in

of(some_pattern(pattern_spec,:binding) {|matched_value|
  some_more_test_on(matched_value)
})

Case#_

This is the wildcard that matches anything

of(_)

You can use binding to say "I don't care what this and that are, but I want them to be the same", like so,

of([_(:a),1,_(:a)])

The pattern matcher binds whatever matches the first and last element of the array to ":a", the binding fails unless those elements are the same.

Case#a

Matches any object of a class.

of(a(String)) 

Case#literal

Matches an object by its == method.

of(literal(1))
of(literal(:b))

Case#integer

Matches an integer.

of(integer)              # any integer
of(integer(1))           # matches 1
of(integer(1..10))       # matches a range
of(integer([1,3,5,7,9])) # matches any number in the set

Case#string

of(string)
of(string("abc"))
of(string(/abc/))

Case#symbol

of(string)
of(string(:abc))
# matches a symbol if its to_s matches the regexp.
of(string(/abc/))

Structural Patterns

For "container" objects like arrays and hashes, pattern matching is a nice way to specify the properties of the object, and taking it apart.

Case#array

of([])    # == of(array([]))
of([1,2]) # matches exactly [1,2]
of([symbol,string,symbol])

# matches an array the first and last element are
# the same symbol, the second element is a string
of([symbol(nil,:a),string,symbol(nil,:a)])

You can match the tail of an array by using the special method Case#_!

c = Case.new {
 of([1,2,_!(integer)])
}
c.match([1])         # Case::NoMatch
c.match([1,2])       # => true
c.match([1,2,3])     # => true
c.match([1,2,3,4,5]) # => true
c.match([1,2,3,:a])  # Case::NoMatch

this pattern matches any array whose first two elements are 1 and 2, the rest are integers.

Case#hash

Hashes are pattern matched by specifying the what pattern a key's value should match. A key can be either be required, or optional, for the pattern matching to succeed.

c = Case.new {
 of("required" => integer, ["optional"] => string)
}
c.match("required" => 10, "optional" => "a") # => true
c.match("required" => 10, "optional" => 1)   # Case::NoMatch
c.match("required" => 10)                    # => true
c.match("required" => "a")                   # => Case::NoMatch

The optional key is denoted by wrapping that in an array.

Case#one_of

This pattern matches if any one of its array of patterns matches.

c = Case.new {
 of(one_of([integer,string]))
}
c.match(10)    # => true
c.match("yay") # => true
c.match(:foo)  # => Case::NoMatch

Higher Order Pattern

The pattern matcher is just constructed with a bunch of objects. So it's possible to save a pattern in a variable, and use that to build larger patterns.

Build a two element array of a custom pattern,

c = Case.new {
 my_pattern = [symbol,string]
 of([my_pattern,my_pattern])
}

Generate pattern with lambda,

c = Case.new {
 pattern = lambda {|pattern1,pattern2| [pattern1,pattern2]}
 of(pattern.call(symbol,string))
 of(pattern.call(string,symbol))
}

Case#is

This method is used to coerce an object into pattern if it isn't already.

is(/regexp/)   # string(regexp)
is(some_class) # matches by class
is(obj)        # literal(obj) for any object

Mostly useful for higher order stuff you are doing.

Case#bind

This is used to matched values to variables. It takes a pattern, and a binding name, and possibly a guard.

c = Case.new {
 foo = is([string])
 of(bind(foo,:a)) { [1,a] }
 of(bind([foo],:a)) { [2,a] }
}
c.match(["a"])   # => [1,["a"]]
c.match([["a"]]) # => [2,[["a"]]]