# Mimic functionality of a macro starting from an expression

**URL:** <https://discourse.julialang.org/t/mimic-functionality-of-a-macro-starting-from-an-expression/20953>\
**Category:** New to Julia\
**Created:** [February 18, 2019, 10:15pm UTC](https://discourse.julialang.org/t/mimic-functionality-of-a-macro-starting-from-an-expression/20953 "2019-02-18T22:15:42Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![isaacsas](https://avatars.discourse-cdn.com/v4/letter/i/f6c823/32.png) [@isaacsas](https://discourse.julialang.org/u/isaacsas)\
**Post date:** [February 18, 2019, 10:15pm UTC](https://discourse.julialang.org/t/mimic-functionality-of-a-macro-starting-from-an-expression/20953/1 "2019-02-18T22:15:42Z")

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Sorry if this is a common question and I just missed the answer when searching around.

I’m trying to mimic the functionality of a macro by building up an expression similar to what it takes as input, calling the same function the macro calls, and then calling `eval` on the resulting expression. I can’t quite seem to make this work, so was hoping for some advice on what to do differently.

We have the following macro in DiffEqBiological:

```julia
macro min_reaction_network(ex::Expr, p...)
    min_coordinate(:min_reaction_network, MacroTools.striplines(ex), p, :no ___noise___ scaling)
end

```

Typical usage would be

```julia
network = @min_reaction_network begin
  c1, X --> 2X
  c2, X --> 0
end c1 c2 

```

My understanding is that in the macro definition `ex` corresponds to the `begin` to `end` block expression, and `p` to a tuple of the symbols at the end `(:c1,:c2,:c3)`.

I’m trying to create a package to read in a different network file format and output a `min_reaction_network`. My approach was to try to build up an equivalent expression to what gets passed into `min_coordinate`, and a similar parameter tuple, and then call `eval` on what `min_coordinate` returns. The issue is that I don’t seem to be able to quite figure out how to build this expression correctly.

For say the example above, can anyone show me how to build an equivalent expression to what the macro passes to `min_coordinate` so that I can call it directly?

Thanks!

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<div class="post-metadata">

**Author:** ![ffevotte](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ffevotte/32/6587_2.png) [@ffevotte](https://discourse.julialang.org/u/ffevotte)\
**Post date:** [February 18, 2019, 10:29pm UTC](https://discourse.julialang.org/t/mimic-functionality-of-a-macro-starting-from-an-expression/20953/2 "2019-02-18T22:29:22Z")

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I’m not sure I understand exactly what you want to do, but maybe this approach would suit you.

First, modify your macro so that it does not call `min_coordinate`, but only build its argument tuple:

```julia
macro min_reaction_network_2(ex::Expr, p...)
    (:min_reaction_network, MacroTools.striplines(ex), p, :no ___noise___ scaling)
end

```

then, expand the macro call (a direct evaluation might work too):

```julia
julia> @macroexpand @min_reaction_network_2 begin
         c1, X --> 2X
         c2, X --> 0
       end c1 c2

(:min_reaction_network, quote
    (c1, $(Expr(:-->, :X, :(2X))))
    (c2, $(Expr(:-->, :X, 0)))
end, (:c1, :c2), :no ___noise___ scaling)

```

this tuple should be what the macro passes to `min_coordinates`

---

<div class="post-metadata">

**Author:** ![isaacsas](https://avatars.discourse-cdn.com/v4/letter/i/f6c823/32.png) [@isaacsas](https://discourse.julialang.org/u/isaacsas)\
**Post date:** [February 18, 2019, 10:49pm UTC](https://discourse.julialang.org/t/mimic-functionality-of-a-macro-starting-from-an-expression/20953/3 "2019-02-18T22:49:13Z")

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Thanks for the help. I’d like to be able to do something like:

```julia
myexpr = min_coordinate(...)
eval(myexpr)

```

which I was hoping would mimic the functionality of the macro.

Using your tuple I still don’t seem to be able to do this:

```julia
mcargs = (:min_reaction_network, quote
    (c1, $(Expr(:-->, :X, :(2X))))
    (c2, $(Expr(:-->, :X, 0)))
end, (:c1, :c2), :no ___noise___ scaling)
myexpr = DiffEqBiological.min_coordinate(mcargs[1],MacroTools.striplines(mcargs[2]),mcargs[3],mcargs[4])

```

returns an expression, but calling `eval(myexpr)` gives an invalid syntax error. Is this somehow related to the expression not being evaluated within the DiffEqBiological package?

---

<div class="post-metadata">

**Author:** ![isaacsas](https://avatars.discourse-cdn.com/v4/letter/i/f6c823/32.png) [@isaacsas](https://discourse.julialang.org/u/isaacsas)\
**Post date:** [February 18, 2019, 10:55pm UTC](https://discourse.julialang.org/t/mimic-functionality-of-a-macro-starting-from-an-expression/20953/4 "2019-02-18T22:55:36Z")

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At a higher level, I am trying to avoid building a giant string that contains the full macro command and just calling `Meta.parse` and then `eval` on it. i.e. avoid:

```julia
str = "@min_reaction_network begin
  c1, X --> 2X
  c2, X --> 0
end c1 c2 "
network = eval(Meta.parse(str))

```

That approach works fine, but I was hoping to just go directly from expressions to getting the `network` out.

If there is a different approach that makes more sense to you please do let me know.