functional programming - Standard ML: Operator and Operand Don't Agree (Circularity) -

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i'm trying write function in sml flip alternate elements of list. here's function:

 fun flipalternate(nil) = nil      | flipalternate([x]) = x      | flipalternate(x::y::xs) = y::x::flipalternate(xs);

when go use file (ullman.sml) in interactive interpreter, compilation error:

- use "ullman.sml"; [opening ullman.sml] ullman.sml:5.31-5.54 error: operator , operand don't agree [circularity]   operator domain: 'z list * 'z list list   operand:         'z list * 'z list   in expression:     x :: flipalternate xs

so sml saying requires list of lists of integers i'm giving pair of lists of integers?

i'm bit lost here appreciated.

thanks, bclayman

your second case wrong; want

fun flipalternate(nil) = nil  | flipalternate([x]) = [x]  | flipalternate(x::y::xs) = y::x::flipalternate(xs);

sml looking @ second case , concluding

flipalternate :: 'z list list -> 'z list

which isn't compatible recursion in third case.

edit: knows result list first case, , concludes argument has 1 more list result second case.


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