Szathmáry László honlapja @ DEIK | Nim2 / anonymous tuples

In Python, a tuple is immutable. In Nim, it depends on how you decared it: with var (mutable) or with let (immutable). The size of a tuple is fix.

import typetraits # echo n.type.name

var t = ("Banana", 2, 'c')

echo t # ("Banana", 2, 'c')

echo t.type.name # (string, int, char) <-- this is its type

echo t[0] # Banana

#echo t[^1] # Error: it won't return the last element :(

t[0] = "Kiwi"
echo t # ("Kiwi", 2, 'c')

# echo t.len # Error: len doesn't work with tuples :(

echo tupleLen(typeof(t)) # 3; number of elements

Receiving a tuple in a function

When we pass a tuple to a function, how to receive it?

proc show_last_column(t: tuple[a: string, b: string, c: string, d: int]) =
echo t.d

let tup = ("001", "Albany", "NY", 162692)
show_last_column(tup)

Or, alternatively:

proc show_last_column(t: (string, string, string, int)) =
echo t[3]

let tup = ("001", "Albany", "NY", 162692)
show_last_column(tup)

Return multiple values from a function

Like in Python:

func getMovieInfo(): (string, int, float) =
return ("Total Recall", 1990, 7.5)

let t = getMovieInfo()

echo t # ("Total Recall", 1990, 7.5)
echo t[1] # 1990

let (title, year, score) = getMovieInfo() # !!! on the left side, use parentheses !!!

echo title # Total Recall
echo year # 1990
echo score # 7.5

Gotcha

Nim has a surprising behaviour that can be strange after Python:

var a, b = 3
#[ it's short for:
var a = 3
var b = 3
]#
echo a, " ", b # 3 3

var x, y = f()
#[ short for:
var x = f()
var y = f()
That is: f() is called twice!
]#

let title, year, score = getMovieInfo()
#[ short for:
let title = getMovieInfo()
let year = getMovieInfo()
let score = getMovieInfo()
Ooops! This is not what we wanted.
]#

If you want to do value unpacking from a tuple, don't forget the parentheses around the variables:

let (title, year, score) = getMovieInfo() # !!! on the left side, use parentheses !!!

Iterate over the values in a tuple

The len() function doesn't work on tuples (see above). If you just want to iterate over the values in a tuple, you can do this:

let t = (1, "hello", 3.14)

for value in t.fields:
echo value

Output:

1
hello
3.14

Anonymous tuples are comparable

let
ta = (2, 8)
tb = (2, 8)
tc = (3, 12)

echo ta == tb # true

echo ta == tc # false