问题
I would like to define a function in Isabelle/HOL that doubles a list
fun double :: "'a list => 'a list" where
...
such that double [x1, x2, ...] = [x1, x1, x2, x2, ...]
I have tried the following:
fun double :: " 'a list ⇒ 'a list" where
"double [] = []" |
"double [x#[l]] = x # x # double [l]"
as well as some other definitions. I get the error
Type unification failed
Type error in application: incompatible operand type
What is wrong with my function?
回答1:
Actually the error message contains some further information. Namely
Operator: double :: 'a list ⇒ 'a list
Operand: [[x, l]] :: ??'a list list
which tells us that [[x, l]] is of type ?'a list list, i.e., a list of lists. As you want to give it as argument to double, which expects an argument of type 'a list, you get the type error.
The origin of the term [[x, l]] in the error message is the second line of your definition
`double [x # [l]]`
where x#[l] is printed as the equivalent [x, l].
There are several superfluous brackets in your input. Note that in contrast to informal mathematical text (with informal meaning as on paper ;)) in Isabelle you cannot use brackets [, ] for explicit nesting. Try the following instead.
fun double :: " 'a list ⇒ 'a list"
where
"double [] = []" |
"double (x#xs) = x # x # double xs"
来源：https://stackoverflow.com/questions/26451510/function-to-double-a-list-in-isabelle