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Update "catemorphisms" article to new math delimiters

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Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>
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Danila Fedorin 2024-05-13 18:59:54 -07:00
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content/blog/haskell_catamorphisms.md
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@ -64,7 +64,7 @@ If we kept going with this process infinitely, we'd eventually have what we need
{{< /latex >}} {{< /latex >}}
But hey, the stuff inside the first set of parentheses is still an infinite sequence of applications But hey, the stuff inside the first set of parentheses is still an infinite sequence of applications
of the function \\(\\text{lengthF}\\), and we have just defined this to be \\(\\text{length}\\). Thus, of the function \(\text{lengthF}\), and we have just defined this to be \(\text{length}\). Thus,
we can rewrite the above equation as: we can rewrite the above equation as:
{{< latex >}} {{< latex >}}
@ -77,7 +77,7 @@ such a fixed point. It's defined like this:
{{< codelines "Haskell" "catamorphisms/Cata.hs" 16 16 >}} {{< codelines "Haskell" "catamorphisms/Cata.hs" 16 16 >}}
This definition is as declarative as can be; `fix` returns the \\(x\\) such that \\(x = f(x)\\). With This definition is as declarative as can be; `fix` returns the \(x\) such that \(x = f(x)\). With
this, we finally write: this, we finally write:
{{< codelines "Haskell" "catamorphisms/Cata.hs" 18 18 >}} {{< codelines "Haskell" "catamorphisms/Cata.hs" 18 18 >}}
@ -95,7 +95,7 @@ This is a valid criticism, so I'd like to briefly talk about how `fix` is used i
lambda calculus. lambda calculus.
In the untyped typed lambda calculus, we can just define a term that behaves like `fix` does. The In the untyped typed lambda calculus, we can just define a term that behaves like `fix` does. The
most common definition is the \\(Y\\) combinator, defined as follows: most common definition is the \(Y\) combinator, defined as follows:
{{< latex >}} {{< latex >}}
Y = \lambda f. (\lambda x. f (x x)) (\lambda x. f (x x )) Y = \lambda f. (\lambda x. f (x x)) (\lambda x. f (x x ))
@ -107,7 +107,7 @@ When applied to a function, this combinator goes through the following evaluatio
Y f = f (Y f) = f (f (Y f)) =\ ... Y f = f (Y f) = f (f (Y f)) =\ ...
{{< /latex >}} {{< /latex >}}
This is the exact sort of infinite series of function applications that we saw above with \\(\\text{lengthF}\\). This is the exact sort of infinite series of function applications that we saw above with \(\text{lengthF}\).
### Recursive Data Types ### Recursive Data Types
We have now seen how we can rewrite a recursive function as a fixed point of some non-recursive function. We have now seen how we can rewrite a recursive function as a fixed point of some non-recursive function.
@ -152,7 +152,7 @@ Looking past the constructors and accessors, we might write the above in pseudo-
newtype Fix f = f (Fix f) newtype Fix f = f (Fix f)
``` ```
This is just like the lambda calculus \\(Y\\) combinator above! Unfortunately, we _do_ have to This is just like the lambda calculus \(Y\) combinator above! Unfortunately, we _do_ have to
deal with the cruft induced by the constructors here. Thus, to write down the list `[1,2,3]` deal with the cruft induced by the constructors here. Thus, to write down the list `[1,2,3]`
using `MyListF`, we'd have to produce the following: using `MyListF`, we'd have to produce the following:
@ -163,7 +163,7 @@ This is actually done in practice when using some approaches to help address the
it's quite unpleasant to write code in this way, so we'll set it aside. it's quite unpleasant to write code in this way, so we'll set it aside.
Let's go back to our infinite chain of type applications. We've a similar pattern before, Let's go back to our infinite chain of type applications. We've a similar pattern before,
with \\(\\text{length}\\) and \\(\\text{lengthF}\\). Just like we did then, it seems like with \(\text{length}\) and \(\text{lengthF}\). Just like we did then, it seems like
we might be able to write something like the following: we might be able to write something like the following:
{{< latex >}} {{< latex >}}
@ -193,8 +193,8 @@ The two mutual inverses \(f\) and \(g\) fall out of the definition of the <code>
data type! If we didn't have to deal with the constructor cruft, this would be more data type! If we didn't have to deal with the constructor cruft, this would be more
ergonomic than writing our own <code>myIn</code> and <code>myOut</code> functions. ergonomic than writing our own <code>myIn</code> and <code>myOut</code> functions.
{{< /sidenote >}} {{< /sidenote >}}
that take you from one type to the other (and vice versa), such that applying \\(f\\) after \\(g\\), that take you from one type to the other (and vice versa), such that applying \(f\) after \(g\),
or \\(g\\) after \\(f\\), gets you right back where you started. That is, \\(f\\) and \\(g\\) or \(g\) after \(f\), gets you right back where you started. That is, \(f\) and \(g\)
need to be each other's inverses. For our specific case, let's call the two functions `myOut` need to be each other's inverses. For our specific case, let's call the two functions `myOut`
and `myIn` (I'm matching the naming in [this paper](https://maartenfokkinga.github.io/utwente/mmf91m.pdf)). They are not hard to define: and `myIn` (I'm matching the naming in [this paper](https://maartenfokkinga.github.io/utwente/mmf91m.pdf)). They are not hard to define: