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smlsmlnj

how to check if list is empty

just started learning sml so excuse me for any discomfort that i may cause.

Okay so here is my function:

fun swapPairsInList [(x,y)]
swapPairsInList: (’x * ’y) list --> (’y * ’x) list

I know how to swap the pairs in the list (recursively) but where i'm having issues is with the base case on when the list is empty (null). How exactly do i check if this list is null? I tried

null [(x,y)]

but that's just throwing back an exception. Should i be using pattern matching to solve this problem?

Solution

  • Okay so i figured it out, I was attacking the problem from the wrong angle once i looked at the map function located in the ListPair structure

    New code:

    fun swap (x,y) = (y,x);
fun pairSwap l = map swap l;