Haskell 变量范围错误和创建正确类型声明的问题

Question

In most of my functions, I seem to get the common error of "Variable not in scope" For example this function

zipmap :: [Int]->[Int]->[Int]
zipmap x y = map (\(a,b) -> a + b) (x zip y)

would give me "Variable not in scope: zipmap :: [Integer] -> [Integer] -> t"

Why is this happening, and for future reference what can I do to avoid these errors?

Answer 1

My guess is that there are already errors when you define the function. Since you fail to define the zipmap function, the interactive shell assumes nothing happened, and thus it does not know about a function zipmap later in the process. For example:

$ ghci
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Prelude> zipmap :: [Int]->[Int]->[Int]; zipmap x y = map (\(a,b) -> a + b) (x zip y)


Prelude> zipmap [1,2,3] [1,4,2,5]

<interactive>:2:1: error:
    Variable not in scope: zipmap :: [Integer] -> [Integer] -> t

Note the part in boldface: the interpreter says it does not understand what you say, so it can not proceed with that. Then later if we use zipmap , it raises the same error like the one you describe.

Your zipmap function is wrong since you write:

x zip y

zip :: [a] -> [b] -> [(a,b)] is a function, not an operator, so you should use it as:

zip x y

Or in full:

zipmap :: [Int] -> [Int] -> [Int]
zipmap x y = map (\(a,b) -> a + b) (zip x y)

We can further improve this: there is a zipWith :: (a -> b -> c) -> [a] -> [b] -> [c] function:

zipmap :: [Int] -> [Int] -> [Int]
zipmap x y = zipWith (+) x y

or even shorter:

zipmap :: [Int] -> [Int] -> [Int]
zipmap = zipWith (+)

and finally we can generalize our function:

zipmap :: Num a => [a] -> [a] -> [a]
zipmap = zipWith (+)