[Issue 9801] New: Syntax sugar for self-assignment of this arguments

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=9801

           Summary: Syntax sugar for self-assignment of this arguments
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2013-03-23 12:32:11 PDT ---
This code is repetitive, not DRY, and it's also bug-prone, it repeats names
like x and y four times, and their types two times:


class FooA {
    public int x;
    private int y;

    this(int x_, int y_) {
        this.x = x_;
        this.y = y_;
    }
}


It's bug prone because it's common to write code like this by mistake, that the
D compiler doesn't see as wrong (see Issue 3878 ):

    this(int z_, int w_) {
        z = z_;
        w = w;
    }


So I suggest to enhance D allowign sugar syntax like this, that produces code
equivalent to FooA:

class FooB {
    public int x;
    private int y;

    this(this.x, this.y) {}
}


Other languages like Scala and TypeScript support a related but different
syntax.

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To keep this enhancement simple this is not allowed (the ctor {} are missing):


class Foo2 {
    public int x;
    private int y;

    this(this.x, this.y); // error
}

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Multiple constructors with self-assigning fields are allowed:


class Foo3 {
    public int x;
    private int y;

    this(this.x) {}
    this(this.x, this.y) {}
}

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Default arguments are allowed:


class Foo4 {
    int x;
    this(this.x = 1) {}
}

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C-style variadic arguments and D-style variadic arguments are not allowed to
become automatic assignments.

But this is allowed:


class Foo5 {
    private int[] data;
    this(this.data...) {}
}

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Mixing self-assigning arguments with regular arguments is allowed:


class Foo6 {
    int x, y;

    this(in this.x, in int y_) {
        this.y = x + y_;
    }
}

- - - - - - - - - -

lazy arguments are allowed:


class Foo7 {
    int x;
    this(lazy this.x) {}
}

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For keep the design simple, you can't add a type to self-assigned arguments:


class Foo8 {
    int x;
    this(byte this.x) {} // error
}


But const/immutable/in/lazy/etc are allowed:


class Foo9 {
    int x;
    this(in this.x) {}
}

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The self-assignment syntax is allowed for all struct/class methods, not just
for constructors:


struct Foo10 {
    int x, y;

    int bar(this.x) {
        y += x;
    }
}


It's equivalent to:

struct Foo10 {
    int x, y;

    int bar(int x_) {
        this.x = x_;
        y += x;
    }
}

- - - - - - - - - -

If there is a pre-condition:


struct Foo11 {
    int x, y;

    int bar(this.x)
    in {
        assert(x > 10);
    } body {
        y += x;
    }
}


It's equivalent to this (so the pre-condition changes the struct state):


struct Foo11 {
    int x, y;

    int bar(int x_)
    in {
        this.x = x_;
        assert(x > 10);
    } body {
        y += x;
    }
}


Or to just this when the preconditions are not compiled in:

struct Foo11 {
    int x, y;

    int bar(int x_) {
        this.x = x_;
        y += x;
    }
}

- - - - - - - - - -

To keep the design clean the public/private/package/protected tag in the method
signature aren't allowed:


class Foo12 {
    public int x;

    this(public this.x) {} // error
}

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Free functions are not allowed to use self-assignment syntax, there is no
"this":

int x;
void bar(this.x) {}

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