Safe pattern matching

[Nikita Karetnikov]

Any Haskellers here?

How would you rewrite the following function in Guile?

foo :: [Int] -> String -> [Int]
foo (x:y:ys) "+" = (x + y):ys
foo (x:y:ys) "-" = (x - y):ys
foo xs       num = read num:xs

*Main> foo [] "42"
[42]
*Main> foo [1,2] "42"
[42,1,2]
*Main> foo [1,2] "+"
[3]
*Main> foo [1..10] "42"
[42,1,2,3,4,5,6,7,8,9,10]
*Main> foo [1..10] "+"
[3,3,4,5,6,7,8,9,10]

It will fail if you call it like this:

*Main> foo [] "+"
[*** Exception: Prelude.read: no parse

But let's assume that it's OK.

I guess that 'match' is the answer, but I don't understand how to use
it.

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