# Macro - parse argument type from input string

**URL:** <https://discourse.julialang.org/t/macro-parse-argument-type-from-input-string/24717>\
**Category:** General Usage\
**Created:** [May 29, 2019, 7:03am UTC](https://discourse.julialang.org/t/macro-parse-argument-type-from-input-string/24717 "2019-05-29T07:03:02Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![bernhard](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bernhard/32/2619_2.png) [@bernhard](https://discourse.julialang.org/u/bernhard)\
**Post date:** [May 29, 2019, 7:03am UTC](https://discourse.julialang.org/t/macro-parse-argument-type-from-input-string/24717/1 "2019-05-29T07:03:02Z")

</div>

I am trying to define this function (with “Int” as a string argument) through a macro

```julia
function my_function_name(x::Int)
    return x*x+one(x)
end

```

Here is my best try (which fails).  
I realize this MWE is not meaningful. In my use case, the body of the function will be different (i.e. a different calculation) based on the type of `x` (say the output is x^2 for Int32 and x^3 for UInt8).

```julia
macro myMacro(tt)
    name="my_function_name"    
    out= quote        
        function $(esc(Symbol(name)))(x::$tt)
            return x*x+one(x)
        end
    end #end quote
    return out
end

@macroexpand @myMacro("Int")

@myMacro("Int")

```

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<div class="post-metadata">

**Author:** ![johnh](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/johnh/32/3615_2.png) [@johnh](https://discourse.julialang.org/u/johnh)\
**Post date:** [May 29, 2019, 7:08am UTC](https://discourse.julialang.org/t/macro-parse-argument-type-from-input-string/24717/2 "2019-05-29T07:08:48Z")

</div>

Does typeof() not help here?

typeof(x)==Int32 && output=x^2  
typeof(x)==UInt8 && output=x^3

---

<div class="post-metadata">

**Author:** ![bernhard](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bernhard/32/2619_2.png) [@bernhard](https://discourse.julialang.org/u/bernhard)\
**Post date:** [May 29, 2019, 7:14am UTC](https://discourse.julialang.org/t/macro-parse-argument-type-from-input-string/24717/3 "2019-05-29T07:14:34Z")

</div>

Not for my use case. Certainly I can do `one(typeof(x))`.  
But for my actual use case, the macro pulls parameters from a dataframe generating a polynomial used in a hot inner loop. And these parameters will depend on the type of `x`. Thus the body of the function will be different for each type of `x`.  
I guess there might be other ways to approach my problem setting. But I would still like to know how (or if), I can write a macro which defines the function mentioned in my first snippet.

---

<div class="post-metadata">

**Author:** ![bernhard](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bernhard/32/2619_2.png) [@bernhard](https://discourse.julialang.org/u/bernhard)\
**Post date:** [May 29, 2019, 7:20am UTC](https://discourse.julialang.org/t/macro-parse-argument-type-from-input-string/24717/4 "2019-05-29T07:20:10Z")

</div>

Ok. This does work.  
I guess the question may come back to:  
I can solve a problem by using “if then else”, a dictionary, multiple dispatch, …

Let us assume that I have 80 different types for which I have a different function body.  
I wanted the functions to depend on the type of the input.  
Certainly I can use 80 if conditions to achieve this. Would it be preferable over multiple dispatch?

---

<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:** [May 29, 2019, 8:20am UTC](https://discourse.julialang.org/t/macro-parse-argument-type-from-input-string/24717/5 "2019-05-29T08:20:14Z")

</div>

You have to call your macro with a type, not a string, like this

```julia
julia> @myMacro Int
my_function_name (generic function with 1 method)

julia> my_function_name(3)
10

```

If you really want the macro to accept strings instead, you have to write your macro more like this:

```julia
macro myMacro(tt)
    name="my_function_name"    
    out= quote        
        function $(esc(Symbol(name)))(x::$(Meta.parse(tt)))
            return x*x+one(x)
        end
    end #end quote
    return out
end

```

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<div class="post-metadata">

**Author:** ![bernhard](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bernhard/32/2619_2.png) [@bernhard](https://discourse.julialang.org/u/bernhard)\
**Post date:** [May 29, 2019, 8:21am UTC](https://discourse.julialang.org/t/macro-parse-argument-type-from-input-string/24717/6 "2019-05-29T08:21:34Z")

</div>

Thank you.

I had a mixup with Base.parse and Meta.parse earlier today. I now realize that these are different functions.