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- Subject: Re: [ANN] tamale-1.2 - Erlang-style pattern matching library
- From: LeafStorm <leafstormrush@...>
- Date: Thu, 23 Sep 2010 22:24:17 -0400
On 09/23/2010 07:36 PM, Patrick Mc(avery wrote:
Your library looks interesting. I tried to search for more information
about Erlang regular expressions and what they might be best suited for
but I am encountering a rather obvious problem, all the examples are in
Erlang! A language I cannot program in.
Could you tell me a just little bit about Erlang regular expressions and
why someone would want to use them?
Thanks for sharing your code-Patrick
I don't know much about Erlang, but in functional languages, pattern
matching doesn't refer to matching text like it does in Lua. Instead,
it's matching some value or values against an expression. For example,
the Haskell factorial function.
fac :: Int -> Int
fac 1 = 1
fac n = n * fac (n - 1)
When you call "fac 1", it matches the first case ("fac 1") and evaluates
it. When you call "fac 2", "fac 3", or something else, it doesn't match
"fac 1", so it tries "fac n", which it does match. It binds the given
value to n and evaluates the expression. Similarly, with the quicksort:
quicksort :: (Ord a) => [a] -> [a]
quicksort  = 
quicksort [x] = [x]
quicksort (x:xs) =
let smallerSorted = quicksort [a | a <- xs, a <= x]
biggerSorted = quicksort [a | a <- xs, a > x]
in smallerSorted ++ [x] ++ biggerSorted
The expression on the last pattern is a bit confusing, but the patterns
"For quicksort with an empty list, do this."
"For quicksort with a list of one ordinal x, do this."
"For quicksort with an ordinal x joined to a list of ordinals xs, do
this." (: in Haskell joins the object on the left to the list on the
right, and ++ is sequence concatenation.)
It makes functional programming a lot more convenient and foolproof.
Regards, Matthew "LeafStorm" Frazier