xcode 无法推断通用参数“Self”（swift 4）
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Generic parameter 'Self' could not be inferred (swift 4)
提问by Haiyuan
I want to make a substring extension for string , I tried below 2 ways but unsuccessful:
我想为 string 做一个子字符串扩展，我尝试了以下两种方法但没有成功：
extension String
{
func substringToFirstChar(of char: Character) -> String
{
let pos = self.range(of: String(char))
let subString = self[..<pos?.lowerBound]
return String(subString)
}
}
or
或者
extension String
{
func substringToFirstChar(of char: Character) -> String
{
let pos = self.index(of: char)
let subString = self[..<pos]
return String(subString)
}
}
xcode prompt error: Generic parameter 'Self' could not be inferredat the "let subString = self[.." line.
xcode 提示错误： 无法在“ let subString = self[..”行推断通用参数“Self”。
How to do that ?
怎么做 ？
回答by vadian
range(of/ index(ofreturns an optional. You have to unwrap the optional in the range expression
range(of/index(of返回一个可选的。您必须在范围表达式中解开可选项
extension String
{
func substringToFirstChar(of char: Character) -> String?
{
guard let pos = self.range(of: String(char))?.lowerBound else { return nil }
// or guard let pos = self.index(of: char) else { return nil }
let subString = self[..<pos]
return String(subString)
}
}
alternatively – to avoid the optional – return the unchanged string if there is no match
或者 - 避免可选 - 如果没有匹配，则返回未更改的字符串
extension String
{
func substringToFirstChar(of char: Character) -> String
{
guard let pos = self.range(of: String(char))?.lowerBound else { return self }
// or guard let pos = self.index(of: char) else { return self }
let subString = self[..<pos]
return String(subString)
}
}
