Question

In sml, we define lists of integers or strings as arguments by l::ls which helps us to define lists of arbitrary length and then we can compare with = or > or <. How can we denote tuples in similar manner?
e.g.
I can write,

fun delete(x,l::ls)=if x=l then delete(x,ls) else l::delete(x,ls)

how can I write similarly for tuples?

Note, I even need to compare the individual elements of the tuple: i.e. (a1,b1)>(a2,b2) if b1>b2 so some sortcut that can merely delete like above will not be sufficient.

Tons of Thank You.

Answer 1

You can do pattern matching directly on tuples using the usual form (x, y).

Your delete function works on any 'a list so it is correct for lists of tuples as well. Here is an example which filters a list based on the first values in tuples:

fun deleteByFirst(x0, []) = []
  | deleteByFirst(x0, (x, y)::ls) = 
    if x = x0 
    then deleteByFirst(x0, ls) 
    else (x, y)::deleteByFirst(x0, ls)