Updated MPJ API draft

Also added native monospace fonts for pre and code tags

source/_drafts/the-api.md

@@ -7,15 +7,32 @@ categories:
 - [ Programming, .NET, F# ]
 - [ Series, A Tour of myPrayerJournal ]
 tags:
+- api
+- composition
+- configuration
+- entity
 - f#
+- fish
+- get
 - giraffe
+- handler
 - json
+- kestrel
+- operator
+- patch
+- post
+- rest
+- router
+- routing
+- scott wlaschin
+- spa
+- suave
 ---
 _NOTES:_
 - _This is post 4 in a series; see [the introduction][intro] for all of them, and the requirements for which this software was built._
 - _Links that start with the text "mpj:" are links to the 1.0.0 tag (1.0 release) of myPrayerJournal, unless otherwise noted._
 
-Now that we have a wonderful, shiny, reactive front end, we need to be able to get some data into it. We'll be communicating via <abbr title="JavaScript Object Notation">JSON</abbr> between the app and the server.
+Now that we have a wonderful, shiny, reactive front end, we need to be able to get some data into it. We'll be communicating via <abbr title="JavaScript Object Notation">JSON</abbr> between the app and the server. In this post, we'll also attempt to explain some about the F# language features used as part of the API.
 
 ## The Data
 
@@ -25,15 +42,15 @@ We apply no special JSON transformations, so the fields in these record types ar
 
 ## The URLs
 
-To set the API apart from the rest of the URLs, they all start with `/api`. Request URLs generally follow the form `/request/[id]/[action]`, and there is a separate URL for the journal. Line 54 in `Program.fs` ([mpj:Program.fs][Program.fs]) has the definition of the routes. We used [Giraffe][]'s [Token Router][TR] instead of the traditional one, as we didn't need to support any URL schemes it doesn't. The result really looks like a nice, clean "table of contents" for the routes support by the API. _(While we tried to follow <abbr title="Representational State Transfer">REST</abbr> principles in large part, the REST purists would probably say that it's not quite RESTful enough to claim the name.)_
+To set the API apart from the rest of the URLs, they all start with `/api`. Request URLs generally follow the form `/request/[id]/[action]`, and there is a separate URL for the journal. Line 54 in `Program.fs` ([mpj:Program.fs][Program.fs]) has the definition of the routes. We used [Giraffe][]'s [Token Router][TR] instead of the traditional one, as we didn't need to support any URL schemes it doesn't. The result really looks like a nice, clean "table of contents" for the routes support by the API. _(While we tried to follow <abbr title="Representational State Transfer">REST</abbr> principles in large part, the REST purists would probably say that it's not quite RESTful enough to claim the name. But, hey, we do use `PATCH`, so maybe we'll get partial credit.)_
 
 We aren't done with routes just yet, though. Let's take a look at that `notFound` handler ([mpj:Handlers.fs][Handlers.fs]); it's on line 27. Since we're serving a <abbr title="Single Page Application">SPA</abbr>, we need to return `index.html`, the entry point of the SPA, for URLs that belong to it. Picture a user sitting at `https://prayerjournal.me/journal` and pressing "Refresh;" we don't want to return a 404! Since the app has a finite set of URL prefixes, we'll check to see if one of those is the URL. If it is, we send the Vue app; if not, we send a 404 response. This way, we can return true 404 responses for the inevitable hacking attempts we'll receive (pro tip, hackers - `/wp-admin/wp-upload.php` does not exist).
 
 ## Defining the Handlers
 
-Giraffe uses the term "handler" to define a function that handles a request. Handlers have the signature `HttpFunc -> HttpContext -> Task<HttpContext option>` (aliased as `HttpHandler`), and can be composed via the `>=>` ("fish") operator. The `option` part in the signature is the key in composing handler functions. The `>=>` operator creates a pipeline that sends the output of one function into the input of another; however, if a function fails to return a `None` option for the `HttpContext` parameter, it short-circuits the remaining logic.
+Giraffe uses the term "handler" to define a function that handles a request. Handlers have the signature `HttpFunc -> HttpContext -> Task<HttpContext option>` (aliased as `HttpHandler`), and can be composed via the `>=>` ("fish") operator. The `option` part in the signature is the key in composing handler functions. The `>=>` operator creates a pipeline that sends the output of one function into the input of another; however, if a function fails to return a `Some` option for the `HttpContext` parameter, it short-circuits the remaining logic.<a href="#note-1"><sup>1</sup></a>
 
-The biggest use of that composition in myPrayerJournal is determining if a user is logged in or not. Authorization is also getting its own post, so we'll just focus on the yes/no answer here. The `authorized` handler (line 71) looks for the presence of a user. If it's there, it returns `next ctx`, where `next` is the next `HttpFunc` and `ctx` is the `HttpContext` it received; this results in a `Task<HttpContext option>` which returns `Some`. If the user is not there, though, it returns the `notAuthorized` handler, also passing `next` and `ctx`; however, if we look up to line 67 and the definition of the `notAuthorized` handler, we see that it ignores both `next` and `ctx`, and returns `None`.
+The biggest use of that composition in myPrayerJournal is determining if a user is logged in or not. Authorization is also getting its own post, so we'll just focus on the yes/no answer here. The `authorized` handler (line 71) looks for the presence of a user. If it's there, it returns `next ctx`, where `next` is the next `HttpFunc` and `ctx` is the `HttpContext` it received; this results in a `Task<HttpContext option>` which continues to process, hopefully following the happy path and eventually returning `Some`. If the user is not there, though, it returns the `notAuthorized` handler, also passing `next` and `ctx`; however, if we look up to line 67 and the definition of the `notAuthorized` handler, we see that it ignores both `next` and `ctx`, and returns `None`. However, notice that this handler has some fish composition in it; `setStatusCode` returns `Some` - it has succeeded - but we short-circuit the pipeline immediately thereafter.
 
 We can see this in use in the handler for the `/api/journal` endpoint, starting on line 137. Both `authorize` and the inline function below it have the `HttpHandler` signature, so we can compose them with the `>=>` operator. If a user is signed in, they get a journal; if not, they get a 403.
 
@@ -53,7 +70,12 @@ For our `POST` example, we'll use `Request.addNote`, starting on line 218. It ch
 
 Giraffe enables [Suave][]-like function composition on top of Kestrel and the ASP.NET Core infrastructure. Rather than using the `Startup` class, myPrayerJournal uses a functional configuration strategy. The calls are in `Program.fs` starting on line 115; there are lots of other guides on how to configure ASP.NET Core, so I won't say too much more about it.
 
-Notice, though, line 31. Earlier, we discussed the `>=>` operator and how it worked. This is an example of the `>>` composition operator, which is part of F#. For functions whose output can be run directly into the next function's input, the `>>` operator allows those functions to be composed into a single function. If we were to look at the signature of the composed function within the parentheses, its signature would be `string -> unit`; so, we pass the string "Kestrel" to it, and it runs through and returns `unit`, which is what we expect for a configuration function. `ctx.Configuration.GetSection`'s signature is `string -> IConfigurationSection`; `opts.Configure`'s signature is `IConfiguration -> KestrelConfigurationLoader`; and `ignore`'s signature is `'a -> unit`. Since `IConfigurationSection` implements `IConfiguration`, we can compose these together easily. We also could have written this as:
+Notice, though, line 31. Earlier, we discussed the `>=>` operator and how it worked. This is an example of the `>>` composition operator, which is part of F#. For functions whose output can be run directly into the next function's input, the `>>` operator allows those functions to be composed into a single function. If we were to look at the signature of the composed function within the parentheses, its signature would be `string -> unit`; so, we pass the string "Kestrel" to it, and it runs through and returns `unit`, which is what we expect for a configuration function. Here's how that breaks down:
+- `ctx.Configuration.GetSection`'s signature is `string -> IConfigurationSection`
+- `opts.Configure`'s signature is `IConfiguration -> KestrelConfigurationLoader` (`IConfigurationSection` implements `IConfiguration`)
+- `ignore`'s signature is `'a -> unit`
+
+If this still doesn't make sense, perhaps this will help. The `Configure.kestrel` function could also have been written using the `|>` piping operator instead, with the equivalent code looking like:
 
 {% codeblock lang:fsharp %}
   let kestrel (ctx : WebHostBuilderContext) (opts : KestrelServerOptions) =
@@ -62,12 +84,14 @@ Notice, though, line 31. Earlier, we discussed the `>=>` operator and how it wor
     |> ignore
 {% endcodeblock %}
 
-...which may help you understand how the `>>` operator works.
-
 <p>&nbsp;</p>
 
 That concludes our tour of the API for now, though we'll be looking at it again next time, when we take a deeper dive into authentication and authorization using Auth0.
 
+---
+
+<a name="note-1"><sup>1</sup></a> _Scott Wlaschin has a great post entitled ["Railway Oriented Programming"][ROP] that explains this concept in general, and [the fish operator][ROP-fish] specifically. Translating his definition to Giraffe's handlers, the first function is `switch1`, the `next` parameter is `switch2`, and the `HttpContext` is the `x` parameter; instead of `Success` and `Failure`, the return type utilizes the either/or nature of an option being `Some` or `None`. If you want to understand what makes F# such a great programming model, you'll spend more time on his site than on The Bit Badger Blog._
+
 
 [intro]: /2018/a-tour-of-myprayerjournal/introduction.html "A Tour of myPrayerJournal: Introduction | The Bit Badger Blog"
 [Data.fs]: https://github.com/bit-badger/myPrayerJournal/blob/1.0.0/src/api/MyPrayerJournal.Api/Data.fs "api/Data.fs | myPrayerJournal | GitHub"
@@ -76,3 +100,5 @@ That concludes our tour of the API for now, though we'll be looking at it again
 [TR]: https://github.com/giraffe-fsharp/Giraffe.TokenRouter "Giraffe.TokenRouter | GitHub"
 [Handlers.fs]: https://github.com/bit-badger/myPrayerJournal/blob/1.0.0/src/api/MyPrayerJournal.Api/Handlers.fs "app/Handlers.fs | myPrayerJournal | GitHub"
 [Suave]: https://suave.io
+[ROP]: https://fsharpforfunandprofit.com/posts/recipe-part2/ "Railway oriented programming | F# for Fun and Profit"
+[ROP-fish]: https://fsharpforfunandprofit.com/posts/recipe-part2/#an-alternative-to-bind "An alternative to bind | Railway oriented programming | F# for Fun and Profit"

themes/bit-badger/source/style.css

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