# DiffEqBiological: getting the error "type Symbol has no field head"

**URL:** <https://discourse.julialang.org/t/diffeqbiological-getting-the-error-type-symbol-has-no-field-head/42296>\
**Category:** Biology, Health, and Medicine\
**Tags:** error\
**Created:** [June 30, 2020, 10:04am UTC](https://discourse.julialang.org/t/diffeqbiological-getting-the-error-type-symbol-has-no-field-head/42296 "2020-06-30T10:04:44Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Sanji\_Vinsmoke](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sanji_vinsmoke/32/9015_2.png) [@Sanji\_Vinsmoke](https://discourse.julialang.org/u/Sanji_Vinsmoke)\
**Post date:** [June 30, 2020, 10:04am UTC](https://discourse.julialang.org/t/diffeqbiological-getting-the-error-type-symbol-has-no-field-head/42296/1 "2020-06-30T10:04:44Z")

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I want to make a simple model of enzyme kinetics,but I keep getting some errors  
This is what I tried:

> rn = @reaction\_network begin  
> k\_f, E + S → ES  
> k\_r, ES → E + S  
> k\_p, ES → P  
> end

and I get the error:

> type Symbol has no field head

And when I try this:

> rn = @reaction\_network begin  
> (k\_f,k\_r), E + S ↔ ES  
> k\_p, ES → P
> 
> end

I get

> syntax: invalid identifier name “–\>”

Any help will be highly appreciated.

Thank You in advanced.

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<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [June 30, 2020, 11:02am UTC](https://discourse.julialang.org/t/diffeqbiological-getting-the-error-type-symbol-has-no-field-head/42296/2 "2020-06-30T11:02:05Z")

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Julia v1.4?

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<div class="post-metadata">

**Author:** ![Sanji\_Vinsmoke](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sanji_vinsmoke/32/9015_2.png) [@Sanji\_Vinsmoke](https://discourse.julialang.org/u/Sanji_Vinsmoke)\
**Post date:** [June 30, 2020, 11:08am UTC](https://discourse.julialang.org/t/diffeqbiological-getting-the-error-type-symbol-has-no-field-head/42296/3 "2020-06-30T11:08:17Z")

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julia version 1.4.2

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<div class="post-metadata">

**Author:** ![simeonschaub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/simeonschaub/32/216566_2.png) [@simeonschaub](https://discourse.julialang.org/u/simeonschaub)\
**Post date:** [June 30, 2020, 11:10am UTC](https://discourse.julialang.org/t/diffeqbiological-getting-the-error-type-symbol-has-no-field-head/42296/4 "2020-06-30T11:10:46Z")

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It looks like your hyphen is an elongated hyphen and therefore a different character, which Julia doesn’t recognize as syntax. Perhaps try editing `<–>` to `<->`.

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<div class="post-metadata">

**Author:** ![Sanji\_Vinsmoke](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sanji_vinsmoke/32/9015_2.png) [@Sanji\_Vinsmoke](https://discourse.julialang.org/u/Sanji_Vinsmoke)\
**Post date:** [June 30, 2020, 11:35am UTC](https://discourse.julialang.org/t/diffeqbiological-getting-the-error-type-symbol-has-no-field-head/42296/5 "2020-06-30T11:35:04Z")

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> m = @reaction\_model begin  
> (k\_f,k\_r), A + B ↔ C  
> end

This also gives

> syntax: invalid identifier name “-\>”

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<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:** [June 30, 2020, 9:43pm UTC](https://discourse.julialang.org/t/diffeqbiological-getting-the-error-type-symbol-has-no-field-head/42296/6 "2020-06-30T21:43:21Z")

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You need to list the parameters at the end like:

```julia
rn = @reaction_network begin
k_f, E + S --> ES
k_r, ES --> E + S
k_p, ES --> P
end k_f k_r k_p

```

If you want to use arrows for reversible reactions they need to be unicode arrows, arrows made from mixing text symbols like `<->` don’t work, see [https://github.com/SciML/DiffEqBiological.jl/issues/198](https://github.com/SciML/DiffEqBiological.jl/issues/198)