Variable capture in block closure

Hello all,

Is it expected that

        | blocks x |

        blocks := OrderedCollection new.

        (1 to: 3) do: [:n |
                x := n.
                blocks add: [Transcript showln: x]].

        blocks do: #value


prints 3 three times, while

        | blocks |

        blocks := OrderedCollection new.

        (1 to: 3) do: [:n |
                | x |
                x := n.
                blocks add: [Transcript showln: x]].

        blocks do: #value

prints 1, 2, 3 ?

Stef

just curious - which of those surprises you?

Hello all,

Is it expected that

    | blocks x |

    blocks := OrderedCollection new.

    (1 to: 3) do: [:n |
        x := n.
        blocks add: [Transcript showln: x]].

    blocks do: #value


prints 3 three times, while

    | blocks |

    blocks := OrderedCollection new.

    (1 to: 3) do: [:n |
        | x |
        x := n.
        blocks add: [Transcript showln: x]].

    blocks do: #value

prints 1, 2, 3 ?

Stef

Hi Stef,

On 1/21/19 5:47 PM, Stéphane Rollandin wrote:
I would expect to see that, yes. I expect the first snippet to update a
single `x`-variable, while the second snippet creates (and then
immediately updates) a fresh `x`-variable within the scope of each
execution of the block argument to `#do:`.

I'd also expect

     | blocks |
     blocks := OrderedCollection new.
     (1 to: 3) do: [:n | blocks add: [Transcript showln: n]].
     blocks do: #value

to print 1, 2, 3, since I would expect a fresh `n` for each invocation
of the `#do:` block.

But then my mind has been poisoned by decades of exposure to high levels
of ambient Scheme.

Regards,
  Tony

> just curious - which of those surprises you?

The first one. I expected the block to capture the variable value at the
block creation time, but from Tony's answer I now see why it does not
work that way.

Stef

Hi Stef,

On Jan 21, 2019, at 10:43 AM, Stéphane Rollandin <[hidden email]> wrote:

>> just curious - which of those surprises you?
>
> The first one. I expected the block to capture the variable value at the block creation time, but from Tony's answer I now see why it does not work that way.

But the second example *does* capture x at block creation time. In the second example a single block is evaluated three times, each time creating a new x (x being a variable that comes into existence when the block is evaluated) and then three times a block nested within it is created, capturing x at block creation time.

>
> Stef
>

Hi Stef,

On 1/21/19 5:47 PM, Stéphane Rollandin wrote:

Is it expected that

    | blocks x |

    blocks := OrderedCollection new.

    (1 to: 3) do: [:n |

        x := n.

        blocks add: [Transcript showln: x]].

    blocks do: #value

prints 3 three times, while

    | blocks |

    blocks := OrderedCollection new.

    (1 to: 3) do: [:n |

        | x |

        x := n.

        blocks add: [Transcript showln: x]].

    blocks do: #value

prints 1, 2, 3 ?

I would expect to see that, yes. I expect the first snippet to update a
single `x`-variable, while the second snippet creates (and then
immediately updates) a fresh `x`-variable within the scope of each
execution of the block argument to `#do:`.

I'd also expect

    | blocks |
    blocks := OrderedCollection new.
    (1 to: 3) do: [:n | blocks add: [Transcript showln: n]].
    blocks do: #value

to print 1, 2, 3, since I would expect a fresh `n` for each invocation
of the `#do:` block.

As should