haskell中的字符串vs字符串？

Question

For type declarations as here , String is used:

p_pair :: CharParser () (String, Maybe String)

And in some other places as here , string is used:

req = ctor <$ string name <* char ' '

Note the capitalization difference between String and string

What is the difference between String and string in Haskell?

Answer 1

In Haskell value and type variable names start with a lower-case letter while value and type constructor names, type synonyms, and classes start with a capital letter. For example:

data Tree a = Empty
            | Node a (Tree a) (Tree a)

height Empty = 0
height (Node _ l r) = max (height l) (height r) + 1

Note that Tree is a type constructor name, Empty and Node are value constructor names and height is a function name. In your case String is the name of the type "string" which is simply a synonym for [Char] , while string is a function.

Answer 2

Looks like string is the function from Text.Parsec.Char while String is the usual Type.

In fact, in the same chapter of RealWorldHaskell , this function is explicitly mentioned under 'Choices and Errors;:

This must be done carefully. Recall that our earlier definition of eol was simply char '\\n'. There is a parser called string that we can use to match the multi-character patterns. Let's start by thinking of how we would add support for \\n\\r

Answer 3

一个是类型 ，另一个是函数 。