Complete updates to Functor chapter

Author: noelwelsh

src/pages/functors/contravariant-invariant.typ

@@ -1,4 +1,4 @@
-#import "../stdlib.typ": info, warning, solution, exercise
+#import "../stdlib.typ": href, info, warning, solution, exercise
 == Contravariant and Invariant Functors 
 <sec:functors:contravariant-invariant>
 
@@ -256,10 +256,10 @@ a pair of bidirectional transformations.
 
 The most intuitive examples of this are a type class
 that represents encoding and decoding as some data type,
-such as Circe's [`Codec`][link-circe-codec]
-and Play JSON's [`Format`][link-play-json-format].
+such as Circe's #href("https://github.com/circe/circe/blob/series/0.14.x/modules/core/shared/src/main/scala/io/circe/Codec.scala")[`Codec`]
+and Play JSON's #href("https://www.playframework.com/documentation/2.6.x/ScalaJsonCombinators#Format")[`Format`].
 We can build our own `Codec` by enhancing `Display`
-to support encoding and decoding to/from a `String`:
+to support encoding and decoding to and from a `String`:
 
 ```scala mdoc:silent
 trait Codec[A] {
@@ -333,9 +333,8 @@ If we want to model more sophisticated relationships
 we can move beyond functors
 to look at _lenses_ and _optics_.
 
-Optics are beyond the scope of this book.
-However, Julien Truffaut's library
-[Monocle][link-monocle] provides a great
+Optics are beyond the scope of this book. However, the
+#href("https://github.com/optics-dev/Monocle")[Monocle] provides a great
 starting point for further investigation.
 ]
 
@@ -426,14 +425,12 @@ encode(Box(123.4))
 decode[Box[Double]]("123.4")
 ```
 
-#warning[
-*What's With the Names?*
-
+#info(title: [What's With the Names?])[
 What's the relationship between the terms
 "contravariance", "invariance", and "covariance"
 and these different kinds of functor?
 
-If you recall from @sec:variance,
+As we discussed in @sec:variance,
 variance affects subtyping,
 which is essentially our ability to use a value of one type
 in place of a value of another type

src/pages/functors/partial-unification.typ

@@ -1,4 +1,4 @@
-#import "../stdlib.typ": info, warning, solution
+#import "../stdlib.typ": href, info, warning, solution
 == Aside: Partial Unification 
 <sec:functors:partial-unification>
 
@@ -44,7 +44,7 @@ type F[A] = Int => A
 type F[A] = A => Double
 ```
 
-*We* know that the former of these is the correct choice.
+_We_ know that the first one is the correct choice.
 However the compiler doesn't understand what the code means.
 Instead it relies on a simple rule, 
 implementing what is called "partial unification".
@@ -83,7 +83,7 @@ In sbt we would add the compiler flag in `build.sbt`:
 scalacOptions += "-Ypartial-unification"
 ```
 
-The rationale behind this change is discussed in [SI-2712][link-si2712].
+The rationale behind this change is discussed in #href("https://issues.scala-lang.org/browse/SI-2712")[SI-2712].
 ]
 
 
@@ -93,7 +93,7 @@ The rationale behind this change is discussed in [SI-2712][link-si2712].
 
 There are situations where
 left-to-right elimination is not the correct choice.
-One example is the `Or` type in [Scalactic][link-scalactic],
+One example is the `Or` type in #href("http://scalactic.org")[Scalactic],
 which is a conventionally left-biased equivalent of `Either`:
 
 ```scala

src/pages/functors/summary.typ

@@ -1,4 +1,4 @@
-#import "../stdlib.typ": info, warning, solution
+#import "../stdlib.typ": href, info, warning, solution
 == Summary
 
 
@@ -27,13 +27,13 @@ but even then it is rare to use them on their own.
 Functors form a foundational building block of
 several more interesting abstractions that we use all the time.
 In the following chapters we will look at two of these abstractions:
-*monads_ and _applicative functors*.
+*monads* and *applicative functors*.
 
 Functors for collections are extremely important, as they transform each element independently of the rest.
 This allows us to parallelise or distribute
 transformations on large collections,
 a technique leveraged heavily in
-"map-reduce" frameworks like [Hadoop][link-hadoop].
+"map-reduce" frameworks popularized by #href("http://hadoop.apache.org")[Hadoop].
 We will investigate this approach in more detail in the
 map-reduce case study later in @sec:map-reduce.