Even though Elixir attempts its best to provide a safe environment for macros, most of the responsibility of writing clean code with macros falls on developers. Macros are harder to write than ordinary Elixir functions, and it's considered to be bad style to use them when they're not necessary. Write macros responsibly.

Focusdistanceformula

The 200mm lens gives your the greatest working distance from your subject and is excellent for capturing images of subjects that might move away when you get close.

In the example above, even though the macro injects a = 1, it does not affect the variable a defined by the go/0 function. If a macro wants to explicitly affect the context, it can use var!/1:

The code above will work but issue a warning: variable "a" is unused. The macro is overriding the original value and the original value is never used.

Minimum focusdistance

The minimum focusing distance of a lens is the shortest or smallest distance that you can be from your subject and still have your subject in focus. The minimum focusing distance (also called the closest focusing distance) of a lens varies from lens to lens. Lenses also have a maximum reproduction ratio which is the ratio of the size of the subject on you camera’s sensor compared to how big it is in real life. A lens that has a reproduction ratio of 1:4 means that however big something is in real life, it will be 1/4 the size on your camera’s sensor.

The function receives the arguments and passes them to if/2. However, as we learned in the previous guide, the macro will receive quoted expressions, inject them into the quote, and finally return another quoted expression.

All three of my macro lenses have different minimum focusing distances. The minimum focusing distance of a Nikon 60mm macro is 18.5cm. The Nikon 105mm minimum focusing distance is 31.4cm and the 200mm macro is 48.8cm. These all seem like workable distances. However, the minimum focusing distance of a lens is NOT the distance from the front of the lens to the subject like some people might think.

Even with such guarantees, the developer plays a big role when writing macros responsibly. If you are confident you need to resort to macros, remember that macros are not your API. Keep your macro definitions short, including their quoted contents. For example, instead of writing a macro like this:

This makes your code clearer and easier to test and maintain, as you can invoke and test do_this_that_and_that/3 directly. It also helps you design an actual API for developers that do not want to rely on macros.

Elixir macros have "late resolution". This guarantees that a variable defined inside a quote won't conflict with a variable defined in the context where that macro is expanded. For example:

Focal length formula

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I realize that the Nikon 200mm macro lens is an older lens, but it is a fantastic lens if you are really serious about macro photography. The minimum focusing distance is really what makes it so great. It is my go to lens for skittish subjects like dragonflies.

Look at the two images below. The one on the left was captured with the 60mm lens and the one on the right was captured with the 200mm lens. Look closely at the size of the wing on the right side of the frame. The wing is larger in the image captured with the 200mm lens. Also, the individual patterns on the wing are also larger in the image captured with the 200mm lens.

Macro.expand_once/2 receives a quoted expression and expands it according to the current environment. In this case, it expanded/invoked the Unless.macro_unless/2 macro and returned its result. We then proceeded to convert the returned quoted expression to a string and print it (we will talk about __ENV__ later in this chapter).

Most camera bodies have a symbol of a circle with a line through it to show where the camera’s sensor is located. The minimum focusing distance is measured from this line to the subject.

Elixir provides similar mechanisms for imports and aliases too. This guarantees that a macro will behave as specified by its source module rather than conflicting with the target module where the macro is expanded. Hygiene can be bypassed under specific situations by using macros like var!/2 and alias!/1, although one must be careful when using those as they directly change the user environment.

Minimum focusdistancecalculator

Elixir already provides mechanisms to write your everyday code in a simple and readable fashion by using its data structures and functions. Macros should only be used as a last resort. Remember that explicit is better than implicit. Clear code is better than concise code.

But not all of these macro lenses are equal. This brings us back to the minimum focusing distance of each of these macro lenses.

Variable hygiene only works because Elixir annotates variables with their context. For example, a variable x defined on line 3 of a module would be represented as:

The 60mm macro lens is a small lightweight lens that is relatively inexpensive. On a crop sensor camera, the 60mm lens is like a 90mm lens which is a nice portrait focal length. Also, I use the 60mm lens along with the Nikon ES-2 Film Digitizing Adaptor Set to digitize images I capture with my film cameras like the Nikon F6.

It is important that a macro is defined before its usage. Failing to define a macro before its invocation will raise an error at runtime, since the macro won't be expanded and will be translated to a function call:

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We can define any function and macro we want, including ones that override the built-in definitions provided by Elixir. The only exceptions are Elixir special forms which are not implemented in Elixir and therefore cannot be overridden. The full list of special forms is available in Kernel.SpecialForms.

The other thing to take into account with these different lenses is the focal length. The longer the focal length of a lens, the more compression you will have in your image. When I use a 200mm lens, objects in the background will appear larger than when I use the 60mm lens.

Focaldistancevs focal length

Macros are lexical: it is impossible to inject code or macros globally. In order to use a macro, you need to explicitly require or import the module that defines the macro.

Take note of the second argument to Macro.var/2. This is the context being used and will determine hygiene as described in the next section. Check out also Macro.unique_var/2, for cases when you need to generate variables with unique names.

Many of the functions in the Macro module expect a Macro.Env environment. You can read more about these functions in Macro and learn more about the compilation environment in the Macro.Env.

In order to better understand how macros work, let's create a new module where we are going to implement unless (which does the opposite of if/2), as a macro and as a function:

With this guide, we finish our introduction to macros. The next guide is a brief discussion on DSLs that shows how we can mix macros and module attributes to annotate and extend modules and functions.

I talk more about macro photography, minimum focusing distances and reproduction ratios in this post about capturing macro images outside your front door.

For this guide, we will be using files instead of running code samples in IEx. That's because the code samples will span multiple lines of code and typing them all in IEx can be counter-productive. You should be able to run the code samples by saving them into a macros.exs file and running it with elixir macros.exs or iex macros.exs.

Let’s look at a closeup of the head of the butterfly. The head captured with the 60mm lens looks slightly wider than the head captured with the 200mm lens. Just like when I am capturing a portrait of a person, the a longer focal length will compress the features while the shorter focal lengths will widen the features. There is nothing bad about the head of the butterfly captured with a 60mm lens. Just be aware that there is a slight difference.

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Constructs such as unless/2, defmacro/2, def/2, defprotocol/2, and many others used throughout the Elixir standard library are written in pure Elixir, often as a macro. This means that the constructs being used to build the language can be used by developers to extend the language to the domains they are working on.

Macros are explicit: it is impossible to run a macro without explicitly invoking it. For example, some languages allow developers to completely rewrite functions behind the scenes, often via parse transforms or via some reflection mechanisms. In Elixir, a macro must be explicitly invoked in the caller during compilation time.

Elixir also supports private macros via defmacrop. Like private functions, these macros are only available inside the module that defines them, and only at compilation time.

Hyperfocaldistance

Macro lenses have a reproduction ratio of 1:1. This means that the object is the same size on the camera’s sensor as it is in real life. A macro lens allows you to get close and capture close up images.

__ENV__/0 returns a Macro.Env struct which contains useful information about the compilation environment, including the current module, file, and line, all variables defined in the current scope, as well as imports, requires, and more:

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Notice that the third element in the quoted variable is the atom Sample, instead of nil, which marks the variable as coming from the Sample module. Therefore, Elixir considers these two variables as coming from different contexts and handles them accordingly.

Minimum focusdistancehuman eye

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The following video further explains the ideas of how the minimum focusing distances and focal lengths of different lenses affect your macro photography.

Macros' language is clear: many languages provide syntax shortcuts for quote and unquote. In Elixir, we preferred to have them explicitly spelled out, in order to clearly delimit the boundaries of a macro definition and its quoted expressions.

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Focal length

There are lots of different macro lenses that have a 1:1 reproduction ratio. All three of my macro lenses (60mm, 105mm and 200mm) can capture images with a 1:1 reproduction ratio. See the images below from each lens.

That's what macros are all about. They are about receiving quoted expressions and transforming them into something else. In fact, unless/2 in Elixir is implemented as a macro:

Check out this video which quickly illustrates reproduction ratio and the advantage that smaller sensors have when it comes to reproduction ratio.

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In our macro implementation, the sentence was not printed, although it was printed in our function implementation. That's because the arguments to a function call are evaluated before calling the function. However, macros do not evaluate their arguments. Instead, they receive the arguments as quoted expressions which are then transformed into other quoted expressions. In this case, we have rewritten our unless macro to become an if/2 behind the scenes.

When the 60mm lens is focused at its minimum focusing distance, the front of the lens is less than 4 cm away from the the subject! This might be ok for some things but it definitely makes it harder to approach insects or other living subjects. You are so close to your subject that you might cast a shadow on your subject and you can’t use your lens hood.

The Nikon 105mm macro lens is a legendary macro lens. It is a great focal length and I use it for macro images and portraits. It has Vibration Reduction (VR) and a fast Silent Wave Motor (SWM). One of the things I like about the 105mm macro compared to the 105mm F1.4 when it comes to portraits is that I can get really close to my subject. The minimum focusing distance of the 105mm F1.4 is 1 meter so I am sometimes further away than I want to be. If I could only have one macro lens, it would be the 105mm macro.

Macros are hygienic: by default, variables defined inside a macro are not going to affect the user code. Furthermore, function calls and aliases available in the macro context are not going to leak into the user context.