7We've used predefined marks already, but we haven't yet created
8our own marks. Let's write a sample mark of our own, then chain
9it together with some preexisting ones to have fun with type
12Let's make a small "cord" mark. "Cord" is a name we use for `@t`
13text, but there's no predefined mark for it. This is
21 ++ atom |=(arg=@ `@t`(scot %ud arg))
30Let's go through this line by line. `/? 314` is the required
31version number, just like in apps. After that everything's in a
32`|_`, which is a `|%` core, but with input arguments. In our
33case, the argument is the marked data.
35There are three possible top-level arms in the `|_` core,
36`++grab`, `++grow`, and `++grad`. `++grad` is used for revision
37control, and it's covered elsewhere. `++grab` specifies
38functions to convert from another mark to the current mark.
39`++grow` specifies how to convert from the current mark to
42In our case, we only define one arm in `++grab`, namely `++atom`.
43This allows us to convert any atom to a cord. `++scot` is a
44standard library function in `hoon.hoon` which pretty-prints
45atoms of various types. We want tot treat the atom as an
46unsigned decimal number, so we give `%ud` and the given argument
47to `++scot`. Thus, the form of an arm in `++grab` is
48`|=(other-mark-type this-mark-value)`.
50In `++grow`, we just convert to the `md` mark, which is just a
51`@t` internally, but it has fancy conversion functions to things
52like html. `++md` isn't a function because the "argument" is the
53`cod=@t` on the `|_` line. We just produce the new value
56Let's play around a bit with this mark. First, let's take a
57marked atom and convert it to our new mark.
60~fintud-macrep:dojo> &atom 9
62~fintud-macrep:dojo> &cord &atom 9
64~fintud-macrep:dojo> &cord &atom &cord &atom 9
68ASCII 9 is 57. There's no requirement, implicit or otherwise,
69that conversions back and forth between marks be inverses of each
70other. These are semantic conversions, in that they refer to the
71same concept. They're not isomorphisms.
73Let's play around a little more:
76~fintud-macrep:dojo> &md &cord &atom 17
78~fintud-macrep:dojo> &hymn &md &cord &atom 17
80 [[%head ~] [[%title ~] [[%~. [%~. "Untitled"] ~] ~] ~] ~]
82 [ i=[g=[n=%meta a=~[[n=%value v="{}"] [n=%name v="frontmatter"] [n=[%urb %front] v=""]]] c=~]
83 t=[i=[g=[n=%p a=~] c=~[[g=[n=%$ a=~[[n=%$ v="17"]]] c=~]]] t=~]
90Ooh, that was exciting. If you squint at that, but it looks an
91awful lot like html. Our value `17` is still in there, in the
92body. `urb` is the mark we render most web pages to. It makes
93sure you have a complete skeleton of a web page. Of course, by
94the time this gets to the web page, it's plain html. Let's do
95the final step in the conversion.
98~fintud-macrep:dojo> &mime &hymn &md &cord &atom 17
101 q='<html><head><title>Untitled</title></head><body><meta value="{}" name="frontmatter" urb:front="" /><p>17</p></body></html>'
105This is a mime-typed octet stream with type `/text/html`, length
106121 bytes, and our lowly number `17` rendered to a web page.
107`cord` was just one step in the chain.
109Of course, arvo can infer some parts of this chain:
112~fintud-macrep:dojo> &cord 17
116Likewise, `md` is the only way to get from `cord` to `hymn`, so
117that can be omitted. If we omit the `hymn`, though, we get:
120~fintud-macrep:dojo> &mime &cord 17
121[[%text %x-markdown ~] p=1 q='7']
124Here, we converted straight from `cord` to `md` to `mime`. Arvo
125decided that was the most straightforward conversion, but it
126gives a different result than passing it through the `hymn`
129The minimal correct input, then, is `&mime &hymn &cord &atom 17`.
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