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Filling an array at compile time
Feb 09, 2022
Vindex
Feb 09, 2022
bauss
Feb 09, 2022
Ali Çehreli
Feb 09, 2022
Ali Çehreli
Feb 10, 2022
bauss
Feb 09, 2022
Anonymouse
Feb 09, 2022
bauss
Feb 09, 2022
user1234
Feb 09, 2022
Vindex
February 09, 2022
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Vindex
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Will the loop (foreach) run at compile time? How can I make it work at compile time?

import std.csv, std.stdio;

alias Record = Tuple!(string, string, string);

immutable string[][] table;

shared static this() {
    string csvText = import("file.csv");
    foreach (record; csvReader!Record(csvText)) {
        table ~= [record[0], record[1], record[2]];
    }
}

void main() {
    writeln(table):
}
February 09, 2022
Gravatar of baussPosted by bauss
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On Wednesday, 9 February 2022 at 08:12:52 UTC, Vindex wrote:

>

Will the loop (foreach) run at compile time? How can I make it work at compile time?

import std.csv, std.stdio;

alias Record = Tuple!(string, string, string);

immutable string[][] table;

shared static this() {
    string csvText = import("file.csv");
    foreach (record; csvReader!Record(csvText)) {
        table ~= [record[0], record[1], record[2]];
    }
}

void main() {
    writeln(table):
}

It will not run at compile-time because csvText is a runtime variable. It should be enum to be accessible at compile-time.

The second issue is that your foreach should probably be static foreach.

The third issue is how you're creating "table". The append operation will be executed at runtime, which means that even if the loop runs at compile-time then you're effectively not winning anything and it basically just becomes a loop-unroll manually done.

The solution would be to create a function that returns a string that is equivalent to the contents of the array you want to create, as if you wrote the content yourself.

And then simply using mixin to "set" the value of the table by calling that function.

Ex.

string getInt() { return "immutable a = 10;"; }

mixin(getInt);

// The variable a is accessible here ...
February 09, 2022
Gravatar of AnonymousePosted by Anonymouse
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On Wednesday, 9 February 2022 at 08:12:52 UTC, Vindex wrote:

>

Will the loop (foreach) run at compile time? How can I make it work at compile time?

import std.csv, std.stdio;

alias Record = Tuple!(string, string, string);

immutable string[][] table;

shared static this() {
    string csvText = import("file.csv");
    foreach (record; csvReader!Record(csvText)) {
        table ~= [record[0], record[1], record[2]];
    }
}

void main() {
    writeln(table):
}

I would do this.

import std;

alias Record = Tuple!(string, string, string);

static immutable string[][] table = () {
    string[][] table;
    string csvText = import("file.csv");

    foreach (record; csvReader!Record(csvText)) {
        table ~= [record[0], record[1], record[2]];
    }

    return table;
}();

pragma(msg, table);  // Available at compile-time

void main() {
    writeln(table);
}

And then -J{path} to tell the compiler where to find file.csv.

dmd -J. csv.d
February 09, 2022
Gravatar of baussPosted by bauss
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On Wednesday, 9 February 2022 at 10:01:15 UTC, Anonymouse wrote:

>

On Wednesday, 9 February 2022 at 08:12:52 UTC, Vindex wrote:

>

Will the loop (foreach) run at compile time? How can I make it work at compile time?

import std.csv, std.stdio;

alias Record = Tuple!(string, string, string);

immutable string[][] table;

shared static this() {
    string csvText = import("file.csv");
    foreach (record; csvReader!Record(csvText)) {
        table ~= [record[0], record[1], record[2]];
    }
}

void main() {
    writeln(table):
}

I would do this.

import std;

alias Record = Tuple!(string, string, string);

static immutable string[][] table = () {
    string[][] table;
    string csvText = import("file.csv");

    foreach (record; csvReader!Record(csvText)) {
        table ~= [record[0], record[1], record[2]];
    }

    return table;
}();

pragma(msg, table);  // Available at compile-time

void main() {
    writeln(table);
}

And then -J{path} to tell the compiler where to find file.csv.

dmd -J. csv.d

Is it guaranteed that the value is initialized at compiletime however?

Something being available at compiletime isn't the same as being initialized at compiletime only.

If it's guaranteed then that's indeed the best solution.

February 09, 2022
Gravatar of VindexPosted by Vindex
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Vindex
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On Wednesday, 9 February 2022 at 10:01:15 UTC, Anonymouse wrote:

>

On Wednesday, 9 February 2022 at 08:12:52 UTC, Vindex wrote:

>

[...]

I would do this.

import std;

alias Record = Tuple!(string, string, string);

static immutable string[][] table = () {
    string[][] table;
    string csvText = import("file.csv");

    foreach (record; csvReader!Record(csvText)) {
        table ~= [record[0], record[1], record[2]];
    }

    return table;
}();

pragma(msg, table);  // Available at compile-time

void main() {
    writeln(table);
}

And then -J{path} to tell the compiler where to find file.csv.

dmd -J. csv.d

Thanks! It does indeed work at compile time.

February 09, 2022
Gravatar of user1234Posted by user1234
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On Wednesday, 9 February 2022 at 10:25:34 UTC, bauss wrote:

>

Is it guaranteed that the value is initialized at compiletime however?

yes, D guarentees this at 100%.

February 09, 2022
Gravatar of Ali ÇehreliPosted by Ali Çehreli
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Ali Çehreli
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On 2/9/22 01:07, bauss wrote:

> It will not run at compile-time because csvText is a runtime variable.
> It should be enum to be accessible at compile-time.

Yes. For the sake of completeness, any expression needed at compile time will be (attempted to be) executed at compile time. For example, an expression used as a template parameter will be executed at compile time as well.

> The append operation
> will be executed at runtime, which means that even if the loop runs at
> compile-time then you're effectively not winning anything and it
> basically just becomes a loop-unroll manually done.

That's not true. It all depends on how the expression is needed. If for example, the variable were defined as enum, the compiler had to execute the code at compile time to compute its value.

> The solution would be to create a function that returns a string [...]
> And then simply using mixin

That is unnecessary and hurts readability. :/ Most programmers see string mixins as a last resort.

In any case, some people may find a compile-time file parsing example that I included in a presentation:

  https://youtu.be/dRORNQIB2wA?t=3157

Ali
February 09, 2022
Gravatar of Ali ÇehreliPosted by Ali Çehreli
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Ali Çehreli
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On 2/9/22 08:37, Ali Çehreli wrote:

> In any case, some people may find a compile-time file parsing example
> that I included in a presentation:

That should mean "may find interesting". And I've just realized that the chapter link is off in that video. This is the beginning of that section:

  https://www.youtube.com/watch?v=dRORNQIB2wA&t=3065s

Ali
February 10, 2022
Gravatar of baussPosted by bauss
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On Wednesday, 9 February 2022 at 16:37:22 UTC, Ali Çehreli wrote:
> On 2/9/22 01:07, bauss wrote:
>
> > It will not run at compile-time because csvText is a runtime
> variable.
> > It should be enum to be accessible at compile-time.
>
> Yes. For the sake of completeness, any expression needed at compile time will be (attempted to be) executed at compile time. For example, an expression used as a template parameter will be executed at compile time as well.
>
> > The append operation
> > will be executed at runtime, which means that even if the
> loop runs at
> > compile-time then you're effectively not winning anything and
> it
> > basically just becomes a loop-unroll manually done.
>
> That's not true. It all depends on how the expression is needed. If for example, the variable were defined as enum, the compiler had to execute the code at compile time to compute its value.
>
> > The solution would be to create a function that returns a
> string [...]
> > And then simply using mixin
>
> That is unnecessary and hurts readability. :/ Most programmers see string mixins as a last resort.
>
> In any case, some people may find a compile-time file parsing example that I included in a presentation:
>
>   https://youtu.be/dRORNQIB2wA?t=3157
>
> Ali

Thank you Ali for the clarifications