# How to construct \`Expr(:tuple, ...)\` using templates

**URL:** https://discourse.julialang.org/t/how-to-construct-expr-tuple-using-templates/1305
**Category:** General Usage
**Tags:** question
**Created:** [January 5, 2017, 3:56pm UTC](https://discourse.julialang.org/t/how-to-construct-expr-tuple-using-templates/1305 "2017-01-05T15:56:38Z")
**Posts on this page:** 1
**Showing post:** 2

<div class="post-metadata">

### Author: ![MikeInnes](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mikeinnes/32/3656_2.png) [@MikeInnes](https://discourse.julialang.org/u/MikeInnes)
#### Post date: [January 5, 2017, 4:39pm UTC](https://discourse.julialang.org/t/how-to-construct-expr-tuple-using-templates/1305/2 "2017-01-05T16:39:54Z")

</div>

When evaluating an expression tree, the only thing that has any meaning is an `Expr` object (to a first approximation at least). Everything else is treated as a black box by the compiler and simply left alone. For example:

```julia
type Foo
  x
end

eval(Expr(:call, :+, :a, Foo(:(b+c))))
# which is equivalent to
a + Foo(:(b+c))
# NOT to
a + Foo(b + c)

```

If you evaluate this expression, `b+c` will never be evaluated, because a `Foo` has to be left as-is. In your case you’re working with a `Tuple` rather than a `Foo` but the principle is exactly the same.

So in your example, what’s happening is that the compiler looks for a set of names to serve as the argument list, but it only finds a value it isn’t allowed to look at – since it can only look inside an `Expr(:tuple)`.

In theory there could be rules for how tuples (or arbitrary objects) are interpreted as expressions so that there’s effectively no difference between `:(a, b)` and `(:a, :b)`. This would make your example behave more intuitively, but would also make the boundary between code and data in `Expr` objects much more complex.

Tuple expressions can be constructed in the normal way by quoting them: `:(a, b)`, `:(foo((1,2)))` etc. And you can splice into them as usual as well; the trick is to start with a one-element tuple expression `:(a,)` and unquote-splat `:($(xs...),)`, making the final version:

```julia
macro_helper1(arguments, form) = :(($(arguments...),) -> $form)

```

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