Home/ Questions/Q 8512487
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-06-11T04:17:30+00:00

This is my solution to exercise from YAHT: Exercise 4.6 Write a datatype Tuple

  • 0

This is my solution to exercise from YAHT:

Exercise 4.6 Write a datatype Tuple which can hold one, two, three or
four elements, depending on the constructor (that is, there should be
four constructors, one for each number of arguments). Also provide
functions tuple1 through tuple4 which take a tuple and return Just the
value in that position, or Nothing if the number is in valid (i.e.,
you ask for the tuple4 on a tuple holding only two elements).

When I wrote a first line I was excited about simplicity comparing to C#


    data Tuplex a b c d = Tuple1 a | Tuple2 a b | Tuple3 a b c | Tuple4 a b c d

    -- class Tuplex<a,b,c,d> {
    --       Tuplex(a p1){ _p1 = p1; }
    --       Tuplex(a p1, b p2){ _p1 = p1; _p2 = p2; }
    --       Tuplex(a p1, b p2, c p3){ _p1 = p1; _p2 = p2; _p3 = p3; }
    --       Tuplex(a p1, b p2, c p3, d p4){ _p1 = p1; _p2 = p2; _p3 = p3; _p4 = p4; }
    --       public Nullable<a> _p1;
    --       public Nullable<b> _p2;
    --       public Nullable<c> _p3;
    --       public Nullable<d> _p4;
    -- }

In C# I can access any field without problem, but here I should write an ‘accessor functions’, right? And amount of code here makes me sad.

Can I have shorter code here?


    tuple1 ∷ Tuplex a b c d → Maybe a
    tuple2 ∷ Tuplex a b c d → Maybe b
    tuple3 ∷ Tuplex a b c d → Maybe c
    tuple4 ∷ Tuplex a b c d → Maybe d
    tuple1 (Tuple1 a) = Just a
    tuple1 (Tuple2 a b) = Just a
    tuple1 (Tuple3 a b c) = Just a
    tuple1 (Tuple4 a b c d) = Just a
    tuple2 (Tuple1 a) = Nothing
    tuple2 (Tuple2 a b) = Just b
    tuple2 (Tuple3 a b c) = Just b
    tuple2 (Tuple4 a b c d) = Just b
    tuple3 (Tuple1 a) = Nothing
    tuple3 (Tuple2 a b) = Nothing
    tuple3 (Tuple3 a b c) = Just c
    tuple3 (Tuple4 a b c d) = Just c
    tuple4 (Tuple1 a) = Nothing
    tuple4 (Tuple2 a b) = Nothing
    tuple4 (Tuple3 a b c) = Nothing
    tuple4 (Tuple4 a b c d) = Just d

    -- unit tests
    prop_tx1 = tuple1 (Tuple1 4) ≡ Just 4
    prop_tx2 = tuple1 (Tuple2 4 'q') ≡ Just 4
    prop_tx3 = tuple2 (Tuple1 4) ≡ (Nothing ∷ Maybe Char)
    prop_tx4 = tuple2 (Tuple2 4 'q') ≡ Just 'q'
Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    I would try to keep it dead simple:

    data Tuplex a b c d = Tuple1 a | Tuple2 a b | Tuple3 a b c | Tuple4 a b c d

toMaybes (Tuple1 p)       = (Just p, Nothing, Nothing, Nothing)
toMaybes (Tuple2 p q)     = (Just p, Just  q, Nothing, Nothing)
toMaybes (Tuple3 p q r)   = (Just p, Just  q, Just  r, Nothing)
toMaybes (Tuple4 p q r s) = (Just p, Just  q, Just  r, Just  s)

tuple1 t = p where (p,_,_,_) = toMaybes t 
tuple2 t = q where (_,q,_,_) = toMaybes t 
tuple3 t = r where (_,_,r,_) = toMaybes t 
tuple4 t = s where (_,_,_,s) = toMaybes t
    • 0