7. What the hell is pattern
matching and why should I
care?
Friday, April 5, 13
8. Wikipedia Definition
“...the act of checking a perceived
sequence of tokens for the presence of
the constituents of some pattern.”
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10. My Best Try
Checking a data type against some
predefined patterns, and when that data
type matches one of the predefined
patterns, do something.
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11. In Short...
Pattern matching is another way
to do control flow.
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31. Pretty easy, and could be implemented in
roughly the same amount of characters with
“if-then-else”
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32. (* Note:
hd(list) takes the first element of list
tl(list) takes all other elements of list
*)
fun sum(numbers) =
if numbers = [] then 0
else hd(numbers) + sum(tl(numbers))
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36. // `~` in Rust is for allocating memory
fn main() {
for int::range(1, 101) |num| {
io::println(
match (num % 3, num % 5) {
(0, 0) => ~"FizzBuzz", // must come first
(0, _) => ~"Fizz",
(_, 0) => ~"Buzz",
(_, _) => int::str(num) // must come last
}
);
}
}
// Source: Lindsey Kuper’s FizzBuzz revisited (http://composition.al/blog/2013/03/02/
fizzbuzz-revisited/)
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37. The real heart of the function:
match (num % 3, num % 5) {
(0, 0) => ~"FizzBuzz", // must come first
(0, _) => ~"Fizz",
(_, 0) => ~"Buzz",
(_, _) => int::str(num) // must come last
}
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38. That’s nice and all, but there’s more...
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39. Both SML and Rust are strongly typed,
and they let us define our own types.
We can then match against those types.
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40. enum Remainder {
zero,
other(NonZeroRem)
}
enum NonZeroRemainder {
one, two, three, four
}
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47. So how about Pattern Matching in Ruby?
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48. pattern-match
github.com/k-tsj/pattern-match
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49. require 'pattern-match'
match(object) do
with(pattern) do
...
end
with(pattern) do
...
end
...
end
# patterns are binded variables
available in block
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50. x = match(0) do
with(String) { “It’s a String!” }
with(Fixnum) { “It’s a Fixnum!” }
end
print x #=> “It’s a Fixnum!”
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51. require 'pattern-match'
1.upto(100) do |n|
match([(n % 3),(n % 5)]) do
with(_[0,0]) { puts "FizzBuzz" }
with(_[0,_]) { puts "Fizz" }
with(_[_,0]) { puts "Buzz" }
with(_[_,_]) { puts n }
end
end
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53. require 'pattern-match'
match([1, "2", 3.0, "four"]) do
with(_[a & 1, b & Or(Float, String), c & Not(Fixnum), d])
with(_[a & Fixnum, b & Not(String), c, d & Not(“four”)])
end
end
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