fix: add missing alt attributes for images, for eBook validation

Author: Delapouite

ch3.md

@@ -102,11 +102,11 @@ From mathisfun.com:
 
 In other words, it's just a relation between two values: the input and the output. Though each input has exactly one output, that output doesn't necessarily have to be unique per input. Below shows a diagram of a perfectly valid function from `x` to `y`;
 
-<img src="images/function-sets.gif" />(http://www.mathsisfun.com/sets/function.html)
+<img src="images/function-sets.gif" alt="function sets" />(http://www.mathsisfun.com/sets/function.html)
 
 To contrast, the following diagram shows a relation that is *not* a function since the input value `5` points to several outputs:
 
-<img src="images/relation-not-function.gif" />(http://www.mathsisfun.com/sets/function.html)
+<img src="images/relation-not-function.gif" alt="relation not function" />(http://www.mathsisfun.com/sets/function.html)
 
 Functions can be described as a set of pairs with the position (input, output): `[(1,2), (3,6), (5,10)]` (It appears this function doubles its input).
 
@@ -115,7 +115,7 @@ Or perhaps a table:
 
 Or even as a graph with `x` as the input and `y` as the output:
 
-<img src="images/fn_graph.png" width="300" height="300" />
+<img src="images/fn_graph.png" width="300" height="300" alt="function graph" />
 
 
 There's no need for implementation details if the input dictates the output. Since functions are simply mappings of input to output, one could simply jot down object literals and run them with `[]` instead of `()`.

ch5.md

@@ -199,7 +199,7 @@ Composition will be our tool for constructing programs and, as luck would have i
 
 Category theory is an abstract branch of mathematics that can formalize concepts from several different branches such as set theory, type theory, group theory, logic, and more. It primarily deals with objects, morphisms, and transformations, which mirrors programming quite closely. Here is a chart of the same concepts as viewed from each separate theory.
 
-<img src="images/cat_theory.png" />
+<img src="images/cat_theory.png" alt="category theory" />
 
 Sorry, I didn't mean to frighten you. I don't expect you to be intimately familiar with all these concepts. My point is to show you how much duplication we have so you can see why category theory aims to unify these things.
 
@@ -213,7 +213,7 @@ In category theory, we have something called... a category. It is defined as a c
 Category theory is abstract enough to model many things, but let's apply this to types and functions, which is what we care about at the moment.
 
 **A collection of objects**
-The objects will be data types. For instance, ``String``, ``Boolean``, ``Number``, ``Object``, etc. We often view data types as sets of all the possible values. One could look at ``Boolean`` as the set of `[true, false]` and ``Number`` as the set of all possible numeric values. Treating types as sets is useful because we can use set theory to work with them. 
+The objects will be data types. For instance, ``String``, ``Boolean``, ``Number``, ``Object``, etc. We often view data types as sets of all the possible values. One could look at ``Boolean`` as the set of `[true, false]` and ``Number`` as the set of all possible numeric values. Treating types as sets is useful because we can use set theory to work with them.
 
 
 **A collection of morphisms**
@@ -224,8 +224,8 @@ This, as you may have guessed, is our brand new toy - `compose`. We've discussed
 
 Here is an image demonstrating composition:
 
-<img src="images/cat_comp1.png" />
-<img src="images/cat_comp2.png" />
+<img src="images/cat_comp1.png" alt="category composition 1" />
+<img src="images/cat_comp2.png" alt="category composition 2" />
 
 Here is a concrete example in code:
 

ch6.md

@@ -132,7 +132,7 @@ app('cats');
 
 This calls our `url` function, then passes the string to our `getJSON` function, which has been partially applied with `trace`. Loading the app will show the response from the api call in the console.
 
-<img src="images/console_ss.png"/>
+<img src="images/console_ss.png" alt="console response" />
 
 We'd like to construct images out of this json. It looks like the srcs are buried in `items` then each `media`'s `m` property.
 
@@ -181,7 +181,7 @@ var app = _.compose(Impure.getJSON(renderImages), url);
 
 And we're done!
 
-<img src="images/cats_ss.png" />
+<img src="images/cats_ss.png" alt="cats grid" />
 
 Here is the finished script:
 ```js