question on map

[Berni44]

On Wednesday, 12 May 2021 at 09:52:52 UTC, Alain De Vos wrote:
> As oppposed to what i expect code below prints nothing nothing 
> on the screen. What is wrong and how to fix it ?
> ```
> import std.stdio;
> import std.range:iota;
> import std.algorithm:map;
>
> bool mywriteln(int x){
> 	writeln(x);
> 	return true;
> }
>
> void main(){
> 	5.iota.map!mywriteln;
> }
>
> ```

You've got a *lazy* range here. Try `5.iota.map!mywriteln.array` 
(you need to import std.array). With that it is not lazy anymore 
and will print.

The way, I'm thinking about lazy ranges is in terms of vouchers, 
like you sometimes have to buy vouchers on sport festivals to get 
sausages. Now you go to the stand with the sausages and hand in 
that voucher, you get the sausage. But if you throw the voucher 
in a waste paper basket, you'll get no sausages, they are not 
even roasted, as long as you do not query them.

What you do above is to buy that voucher and throw it away. 
That's why nothing happens.

So in more detail, `5.iota` gives you a voucher (A) for the 5 
numbers from 0 to 4. You hand that voucher (A) to `map` together 
with a function `mywriteln` and get back a new voucher (B), this 
time for applying the numbers to your function. And that voucher 
(B) is discarded, because it is never used.

If you would write `5.iota.map!mywriteln.front`, you would say 
something like: "Hey `map`, here is my voucher (B), please give 
me the first element you promised to me. `map` now takes the 
voucher (A) you gave it earlier and does the same, it asks 
`iota`: "Hey `iota`, here is my voucher (A), please give me the 
first element." And iota hands `0` to `map`, which now calls 
`mywriteln(0)`. `mywriteln` now prints the expected `0` and hands 
a `true` to `map` and then `map` gives you that `true` (which you 
still discard).

Hope, this analogy helps.