Use tuples when you know in advance how many components some piece of data should have. Tuples are much more rigid because each different size of tuple is its own type, so you can't write a general function to append an element to a tuple — you'd have to write a function for appending to a pair, one function for appending to a triple, one function for appending to a 4-tuple, etc. (查看原文)
In our gcd' function, the logging is fast because the list appending ends up looking like this:
a ++ (b ++ (c ++ (d ++ (e ++ f))))
Lists are a data structure that's constructed from left to right, and this is efficient because we first fully construct the left part of a list and only then add a longer list on the right. But if we're not careful, using the Writer monad can produce list appending that looks like this:
((((a ++ b) ++ c) ++ d) ++ e) ++ f
This associates to the left instead of to the right. This is inefficient because every time it wants to add the right part to the left part, it has to construct the left part all the way from the beginning! (查看原文)
This example introduces Haskell’s if statement. You’re probably already familiar with if statements from other languages, but what makes Haskell’s unique is that the else part is mandatory.
On the other hand, a Haskell program is a collection of functions. Func- tions are used to transform data values into result values, and every function
should return some value, which can in turn be used by another function. Since every function has to return something, this implies that every if has to have a corresponding else. Otherwise, you could write a function that has a return value when a certain condition is met but doesn’t have one when that condition isn’t met! Briefly: Haskell’s if is an expression that must return a value, and not a statement. (查看原文)
Accessing List Elements
If you want to get an element of a list by index, use the !! operator. As with most programming languages, the indices start at 0: (查看原文)
Prelude> let mul3 x = x*x*x
Prelude> :t mul3
mul3 :: Num a => a -> a
Prelude> (mul3 2) 2 3
<interactive>:15:2:
No instance for (Num (a0 -> a1 -> t0))
arising from a use of `mul3'
Possible fix:
add an instance declaration for (Num (a0 -> a1 -> t0))
In the expression: (mul3 2) 2 3
In an equation for `it': it = (mul3 2) 2 3 (查看原文)
Prelude> let iua = (`elem` ['A'..'Z'])
Prelude> iua 'Z'
True
Prelude> iua 1
<interactive>:33:5:
No instance for (Num Char)
arising from the literal `1'
Possible fix: add an instance declaration for (Num Char)
In the first argument of `iua', namely `1'
In the expression: iua 1
In an equation for `it': it = iua 1 (查看原文)
作者: Miran Lipovaca 副标题: A Guide for Beginners isbn: 1593272839 书名: Learn You a Haskell for Great Good! 页数: 400 定价: USD 44.95 出版社: No Starch Press 装帧: Paperback 出版年: 2010
