# RFC: \_ as ans

**URL:** <https://discourse.julialang.org/t/rfc---as-ans/2326>\
**Category:** Internals & Design\
**Created:** [February 27, 2017, 1:06am UTC](https://discourse.julialang.org/t/rfc---as-ans/2326 "2017-02-27T01:06:29Z")\
**Posts on this page:** 20\
**Page:** 1

<div class="post-metadata">

**Author:** ![bramtayl](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bramtayl/32/3614_2.png) [@bramtayl](https://discourse.julialang.org/u/bramtayl)\
**Post date:** [February 27, 2017, 1:06am UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/1 "2017-02-27T01:06:29Z")

</div>

There’s been a lot of discussion lately of using \_ as stand in for the result of the previous evaluation, for example:

```julia
a
_ + 1
_ + 2

```

would get lowered to

```julia
gensym1 = a
gensym2 = gensym1 + 1
gensym3 = gensym2 + 2

```

I’ve got something close to this up and working in ChainRecurisve.jl in a surprisingly small amount of code. There are only two remaining issues:

1. Base syntax that is parsed as a block but would need to special case opt out:

```julia
type a
    b::Int
    c::Int
end

```

does not mean

```julia
type a
    gensym1 = b::Int
    gensym2 = c::Int
end

```

and

```julia
(args...; kwargs...) -> f(args..., kwargs)

```

does not mean

```julia
(begin 
    gensym1 = args...
    gensym2 = kwargs...
end) -> f(args..., kwargs)

```

1. Macros that use block syntax for things that aren’t true blocks. For example:

```julia
MacroTools.@match e begin
    a_ + b_ => :($b + $a)
    a_ => a
end

```

does not mean

```julia
MacroTools.@match e begin
    gensym1 = a_ + b_ => :($b + a)
    gensym2 = a_ => a
end

```

Part of this can be solved in a macro. 1) can be solved using special casing. 2) however cannot because the possibility of user defined macros. Therefore, I think for this to gain widespread usability, it needs to be implemented in lowering, which I gather occurs after macro expansion. I think this is the same way dot vectorization was implemented. I’d like to implement a PR, but I was hoping if anyone is excited about this they could either give me some advice to get started or collaborate? I don’t know lisp so it would be an uphill climb.

---

<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [February 27, 2017, 10:19am UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/2 "2017-02-27T10:19:17Z")

</div>

I was not aware of this discussion, can you link it?

I tend to use `_` as a stand-in for “values I don’t need”, eg

```julia
a, _, c = returns_three_values(...)

```

---

<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [February 27, 2017, 5:32pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/3 "2017-02-27T17:32:26Z")

</div>

The way forward to make that official has been paved by deprecating the use of `_` as an r-value in 0.6. It will likely be an official “discard this value” name in 1.0 when used as a l-value. That does not conflict with the proposed use here as an automatic “last value” binding, which would be handy for chaining computations together.

EDIT: To clarify, there has not been that much discussion of `_` as a previous value binding, most of the discussion has been about `_` as a discarded value binding.

---

<div class="post-metadata">

**Author:** ![bramtayl](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bramtayl/32/3614_2.png) [@bramtayl](https://discourse.julialang.org/u/bramtayl)\
**Post date:** [February 27, 2017, 6:09pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/4 "2017-02-27T18:09:24Z")

</div>

So it’s possible that if \_ is used as an l-value it would mean “discard this value” but it used as a r-value it would mean “last value”?

---

<div class="post-metadata">

**Author:** ![bramtayl](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bramtayl/32/3614_2.png) [@bramtayl](https://discourse.julialang.org/u/bramtayl)\
**Post date:** [February 27, 2017, 6:24pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/5 "2017-02-27T18:24:33Z")

</div>

In fact, these use cases are entirely consistent. For example,

```julia
plus_neighbor(i) = i, i + 1

begin 
    1
    _, b = plus_neighbor(_)
    b + 1
end

```

would presumably get lowered to

```julia
plus_neighbor(i) = i, i + 1

begin 
    gensym1 = 1
    gensym2 = (gensym1, b) = plus_neighbor(gensym1)
    gensym3 = b + 1
end

```

And in fact this already works in ChainRecursive

```julia
using ChainRecursive

@chain begin
    1
    _, b = plus_neighbor(_)
    b + 1
end

```

---

<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [February 27, 2017, 7:25pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/6 "2017-02-27T19:25:55Z")

</div>

> [@bramtayl](#):
>
> There’s been a lot of discussion lately of using \_ as stand in for the result of the previous evaluation, for example:

As Stefan says, there actually hasn’t been much (if any) discussion of this. The discussion has been for using `_` for discarded l-values.

I’m very skeptical of this proposal for `_` as an r-value denoting the result of the previous expression. What problem does this solve? Can you give an example of code that would be made significantly clearer by using `_` to denote the result of a previous expression?

---

<div class="post-metadata">

**Author:** ![bramtayl](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bramtayl/32/3614_2.png) [@bramtayl](https://discourse.julialang.org/u/bramtayl)\
**Post date:** [February 27, 2017, 7:34pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/7 "2017-02-27T19:34:50Z")

</div>

Hmm. Well, I’ve seen it mentioned a couple places, maybe I was exaggerating a bit. I’m not sure I want to rehash the discussion of the pros/cons of chaining; I was more looking for advice on how to implement one version of it.

---

<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [February 27, 2017, 7:41pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/8 "2017-02-27T19:41:20Z")

</div>

Assigning a symbol different meaning depending on context would lead to subtle bugs IMO. Generally, I don’t think that using values of previous expressions via some special syntax is good practice; I recognize that `ans` is useful in the REPL (in case the computation is expensive and I forgot to assign the result to something, but this almost never happens), but I would prefer explicitly assigning and using values.

However, in case you think this is useful, it would be great to see an example of a language which does something similar. I have to admit that I can’t think of any.

---

<div class="post-metadata">

**Author:** ![bramtayl](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bramtayl/32/3614_2.png) [@bramtayl](https://discourse.julialang.org/u/bramtayl)\
**Post date:** [February 27, 2017, 7:58pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/9 "2017-02-27T19:58:32Z")

</div>

In fact, I tried to show above that the semantics of \_ as “discard this value” are a strict subset of the semantics of \_ as “last value”. Chaining is wildly popular in R and used extensively in the Hadleyverse.

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [February 27, 2017, 8:07pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/10 "2017-02-27T20:07:29Z")

</div>

> [@bramtayl](#):
>
> the semantics of \_ as “discard this value” are a strict subset of the semantics of \_ as “last value”

No it’s not. What are you going to do with

```julia
_, a = (1, 2)
b = _

```

Should `_` be the last value, or the discarded value

What about

```julia
c = _, a = (1, 2)
b = _

```

Discarding a value has the property that you cannot misuse it, since it’ll be an error (currently a warning) if you do. A magic “last value” does not have that property and is much more error prone.

---

<div class="post-metadata">

**Author:** ![bramtayl](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bramtayl/32/3614_2.png) [@bramtayl](https://discourse.julialang.org/u/bramtayl)\
**Post date:** [February 27, 2017, 9:14pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/11 "2017-02-27T21:14:43Z")

</div>

Hmm, I think the behavior is pretty consistent.

```julia
_, a = (1, 2)
b = _

```

would go to

```julia
gensym1 = nothing
gensym2 = (gensym1, a) = (1, 2)
gensym3 = gensym2

```

```julia
c = _, a = (1, 2)
b = _

```

would go to

```julia
gensym1 = nothing
gensym2 = c = (gensym1, a) = (1, 2)
gensym3 = b = gensym3

```

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [February 27, 2017, 9:39pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/12 "2017-02-27T21:39:51Z")

</div>

> [@bramtayl](#):
>
> gensym1 = nothing  
> gensym2 = (gensym1, a) = (1, 2)  
> gensym3 = gensym2

This is exactly the problem. It’s perfectly valid to assume that it’s

```julia
(gensym1, a) = (1, 2)
c = gensym1

```

instead.

---

<div class="post-metadata">

**Author:** ![bramtayl](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bramtayl/32/3614_2.png) [@bramtayl](https://discourse.julialang.org/u/bramtayl)\
**Post date:** [February 27, 2017, 10:00pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/13 "2017-02-27T22:00:02Z")

</div>

Hmm. I think the first behavior seems more intuitive.

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [February 27, 2017, 10:26pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/14 "2017-02-27T22:26:03Z")

</div>

The second one (I assume this means the one in my post above) is much more consistent with using it as a normal variable.

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [February 27, 2017, 11:24pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/15 "2017-02-27T23:24:06Z")

</div>

Isn’t `_` as an rvalue deprecated?

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [February 27, 2017, 11:55pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/16 "2017-02-27T23:55:50Z")

</div>

The deprecation does free it up for being used as result of previous statement when used as rvalue which is what @bramtayl has been arguing for. The case I posted is just for showing that having it both as discarded value and value of previous statement are in principle non-conflicting but can be quite confusing.

---

<div class="post-metadata">

**Author:** ![bramtayl](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bramtayl/32/3614_2.png) [@bramtayl](https://discourse.julialang.org/u/bramtayl)\
**Post date:** [February 28, 2017, 12:11am UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/17 "2017-02-28T00:11:16Z")

</div>

I can see your point, it does seem confusing. I think the potential for a user to use \_ in the line directly after an assignment is small. If users are assigning names to something, they’ll likely be using these names instead.

---

<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [February 28, 2017, 2:03pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/18 "2017-02-28T14:03:29Z")

</div>

But the point is that allowing `_` as both and r-value and an l-value but one which behaves completely differently from other variables is wildly confusing and adds a ton of corner cases to the language.

---

<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [February 28, 2017, 2:15pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/19 "2017-02-28T14:15:12Z")

</div>

> [@bramtayl](#):
>
> Chaining is wildly popular in R and used extensively in the Hadleyverse.

If you are thinking of the `%>%` operator in R, that is quite different from what you are suggesting, and equivalent or similar functionality is already provided by `|>` and some packages, in particular see

> **[GitHub - MikeInnes/Lazy.jl: I was gonna maintain this package, but then I got...](https://github.com/MikeInnes/Lazy.jl#macros)**
>
> I was gonna maintain this package, but then I got high - GitHub - MikeInnes/Lazy.jl: I was gonna maintain this package, but then I got high

---

<div class="post-metadata">

**Author:** ![bramtayl](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bramtayl/32/3614_2.png) [@bramtayl](https://discourse.julialang.org/u/bramtayl)\
**Post date:** [February 28, 2017, 3:01pm UTC](https://discourse.julialang.org/t/rfc---as-ans/2326/20 "2017-02-28T15:01:56Z")

</div>

Like I showed above, only two corner cases in Base, each of which is caused by Julia using block syntax for lists/arguments. In fact, I was kicking around another proposal to add a new “arguments” block syntax/AST node which would allow you to pass arguments to a function in block form. This alternate proposal would allow macros to work correctly and remove the need for chaining behavior in Base.

[Next page](https://discourse.julialang.org/t/rfc---as-ans/2326.md?page=2)