level 3
pcall不懂，，，，function a(x)
2015年09月09日 23点09分
level 11
![[滑稽]](/static/emoticons/u6ed1u7a3d.png)
我就是翻译了一下
level 14
print( pcall(pcall, ...) )
falsebad argument #1 to '?' (value expected)
2015年09月11日 05点09分
5
falsebad argument #1 to '?' (value expected)
level 14
level 3
chaosink
楼主
Hello, Mr Ierusalimschy
Exercise 8.4: Can you find any value for f such that the call pcall(pcall, f) returns false as its first result?
I don't know clearly the purpose of Exercise 8.4.
Does it want readers understand that 'pcall' only catches the error message and return it but raise an error so that such 'f' doesn't exist?
Or want really readers to find a 'value' for f?
I find two 'forms' for f that make the call 'pcall(pcall, f)' returns false as the first result:
1. print(pcall(pcall, ...))
2. function f() return end; print(pcall(pcall, f()))
But I'm not sure whether '...' and 'f()' are values in Lua. That's why I use 'form' instead.
And I have some other questions while doing Exercise 8.4:
Why the following two code snippets get different results?
1. function f() return end; print(pcall(pcall, f()))
2. function f() return nil end; print(pcall(pcall, f()))
Why the following two code snippets get different results?
1. function f() return end; print(f())
2. function f() return nil end; print(f())
Why the following two code snippets get the same result?
1. function f() return end; a = f(); print(a)
2. function f() return nil end; a = f(); print(a)
Are 'no value' and 'nil' different? And how to distinguish them?
How to make 'type()' in 'function f() return end; print(type(f()))' safe?
Thank you for your book!
2015年09月12日 07点09分
7
Exercise 8.4: Can you find any value for f such that the call pcall(pcall, f) returns false as its first result?
I don't know clearly the purpose of Exercise 8.4.
Does it want readers understand that 'pcall' only catches the error message and return it but raise an error so that such 'f' doesn't exist?
Or want really readers to find a 'value' for f?
I find two 'forms' for f that make the call 'pcall(pcall, f)' returns false as the first result:
1. print(pcall(pcall, ...))
2. function f() return end; print(pcall(pcall, f()))
But I'm not sure whether '...' and 'f()' are values in Lua. That's why I use 'form' instead.
And I have some other questions while doing Exercise 8.4:
Why the following two code snippets get different results?
1. function f() return end; print(pcall(pcall, f()))
2. function f() return nil end; print(pcall(pcall, f()))
Why the following two code snippets get different results?
1. function f() return end; print(f())
2. function f() return nil end; print(f())
Why the following two code snippets get the same result?
1. function f() return end; a = f(); print(a)
2. function f() return nil end; a = f(); print(a)
Are 'no value' and 'nil' different? And how to distinguish them?
How to make 'type()' in 'function f() return end; print(type(f()))' safe?
Thank you for your book!