Syntax for a function with (’a * ’b) list → (’b * ’a) list

Question

I am trying to write a function to swap a pair of tuples inside of a list like this:

- pairSwap [(1, 2), (3, 4), (5, 6);
 [(2,1),(4,3),(6,5)]

I am having a hard time figuring out what I am doing wrong with my syntax while declaring the function. This is what I have so far:

fun pairSwap ((a : 'a, b: 'a) list) : (('a, 'a) list) = 
...
;

Where am I going wrong?

Edit: Solution

fun pairSwap (t : ('a * 'a) list) : ('a * 'a) list = 
  if null t
    then []
  else
    let
      val v = hd t
    in
      (#2 v, #1 v)::pairSwap (tl t)
    end
;

This is not a dupe of the linked question: that one is about a function that maps e.g. `[1,2,3,4]` to `[2,1,4,3]`, whereas this one is about a function that maps e.g. `[(1,2),(3,4)]` to `[(2,1),(4,3)]`. — ruakh, Jan 13 '18 at 22:09
No, not exactly, but both the title of that question and the answers given would also describe this problem. — sshine, Jan 22 '18 at 11:16

score 1 · Answer 1 · answered Jan 10 '18 at 21:40

1

You need to pull the variable out of the pair type annotation:

fun pairSwap (arg : ('a * 'b) list) : ('b * 'a) list = ...;

Note that I've also changed it from what you had to what the title requested: ('a * 'a) list -> ('a * 'a) list to ('a * 'b) list -> ('b * 'a) list

answered Jan 10 '18 at 21:40

Matt

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Updated the question to address another problem – Jordan Jan 10 '18 at 22:00
Got it! Thank you – Jordan Jan 10 '18 at 22:17

score 1 · Accepted Answer · answered Jan 11 '18 at 08:14

Since you have provided a solution in the meantime, here is some feedback on it:

You don't really need the type annotations. You can let type inference do the work.
```
fun pairSwap t = ...
```
You can use null, hd and tl, but consider pattern matching:
```
fun pairSwap [] = []
  | pairSwap ((x,y)::ps) = (y,x)::pairSwap ps
```
This also makes the let-expression redundant.
As Matt points out, the type for this function is ('a × 'b) list → ('b × 'a) list, which means you can also swap pairs where the left side has a different type ('a) than the right side ('b), e.g.:
```
- pairSwap [("hello",42),("world",43)];
val it = [(42,"hello"),(43,"world")] : (int * string) list
```

Thanks for the feedback. This is my first time playing with sml for one of my classes so I was encouraged to use types to help transition towards sml syntax. I will keep this in mind next time. — Jordan, Jan 12 '18 at 18:29

Syntax for a function with (’a * ’b) list → (’b * ’a) list

2 Answers2