I'm trying to call take on the output of flatten. The problem is that take requires a list of a but flatten returns a list of Any. Is there a way to convert between them? Or some other approach I'm supposed to take? I wasn't able to find any examples in the Racket Docs.
(: extend (All (a) (-> (Listof a) Integer (Listof a))))
(define (extend l n)
; extend a list to length 'n' by appending it to itself
;
; @l list of any
; @n int
; @return list of any
(take (flatten (make-list n l)) n))
From the interpreter, here are the exact types for each function for reference.
> take
- : (All (a) (-> (Listof a) Integer (Listof a)))
#<procedure:take>
> flatten
- : (-> Any (Listof Any))
#<procedure:flatten>
Here's the error message for reference too.
alg/waterfall.rkt:65:2: Type Checker: Polymorphic function `take' could not be applied to arguments:
Argument 1:
Expected: (Listof a)
Given: (Listof Any)
Argument 2:
Expected: Integer
Given: Integer
Result type: (Listof a)
Expected result: (Listof a)
@Alexis King is right. The flatten function has more complicated behavior that doesn't fit with the type you need. The append* function is simpler, and here it's actually what you need instead of flatten.
In the place where you use it:
; n : Integer
; l : (Listof a)
(take (flatten (make-list n l)) n)
; expected type: (Listof a)
The input to flatten is a (Listof (Listof a)), and for it to typecheck the output must be a (Listof a). This has to be true *even when a includes lists*.
The function you want is something with the type (Listof (Listof a)) -> (Listof a). Now, does flatten always have that type? No it can't, here's a counterexample:
a = (Listof Integer)
input : (Listof (Listof (Listof Integer)))
input = (list (list (list 1)))
expected output type: (Listof (Listof Integer))
actual output value: (list 1)
Therefore flatten cannot have the type (Listof (Listof a)) -> (Listof a). What you need is append*, which does have that type.
> append*
- : (All (a) (-> (Listof (Listof a)) (Listof a)))
#<procedure:append*>
In your example, you can use append* where you used flatten.
(: extend (All (a) (-> (Listof a) Integer (Listof a))))
(define (extend l n)
; extend a list to length 'n' by appending it to itself
;
; @l list of any
; @n int
; @return list of any
(take (append* (make-list n l)) n))