输入parsec库所需的签名

Question

以下代码编译没有问题：

import Text.ParserCombinators.Parsec (many, noneOf, GenParser)

notNewline :: GenParser Char st String
notNewline = many (noneOf "\n")

main = print "Hello"

但是一旦我评论了类型签名，就像这样：

import Text.ParserCombinators.Parsec (many, noneOf, GenParser)

--notNewline :: GenParser Char st String
notNewline = many (noneOf "\n")

main = print "Hello"

我收到以下错误：

No instance for (Text.Parsec.Prim.Stream s0 m0 Char)
  arising from a use of `noneOf'
Possible fix:
  add an instance declaration for
  (Text.Parsec.Prim.Stream s0 m0 Char)
In the first argument of `many', namely `(noneOf "")'
In the expression: many (noneOf "")
In an equation for `notNewline': notNewline = many (noneOf "")

为什么我从省略类型签名时收到错误消息？