blog: Add 2025-07-19.md

Author: ingydotnet

blog/2025-07-19.md

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+---
+title: YAML Explicit Keys and YS
+date: 2025-07-19
+draft: false
+authors: [ingydotnet]
+categories: [Summer-of-YS]
+edit: blog/2025-07-19.md
+comments: true
+---
+
+Did you know that in YAML you can write:
+
+```yaml
+? foo
+: bar
+```
+
+instead of:
+
+```yaml
+foo: bar
+```
+
+The `?` is called an explicit key indicator.
+
+But why would you want to use it?
+
+It's not often useful in YAML config files, but it is actually useful in YS
+code.
+
+<!-- more -->
+
+
+## Explicit Keys in YAML
+
+In YAML, a mapping key doesn't have to be a string.
+It can be any node.
+
+In flow mode, we could write:
+
+```yaml
+{ {foo: 1, bar: 2}: baz }
+```
+
+Where `{foo: 1, bar: 2}` is a mapping node that is also a mapping key.
+
+How would we write that in block (indented) mode?
+
+With explicit key syntax!
+
+```yaml
+? foo: 1
+  bar: 2
+: baz
+```
+
+Note that when you use `?` the `:` has to be on a different line and in the same
+column as the `?`.
+
+
+## Explicit Keys in YS
+
+There's another reason to use explicit keys in YAML.
+
+String keys need to be encoded on a single line, even if they are quoted.
+String values can be multi-line, but string keys cannot.
+
+```yaml
+this is a very long string key for example:
+  this is a very
+  long string value
+  for example
+```
+
+It's not a common problem to solve in YAML, but in YS it comes up more often.
+
+In [Yesterday's blog post](2025-07-18.md#the-ys-code) I showed you a YS program
+with a `loop` function that started like this:
+
+```yaml
+    loop i 0 [x y], [0 0], data {}, [minx maxx miny maxy] [0 0 0 0]:
+      dir =: i-to-dir(i)
+      nx ny =: +
+        ...
+```
+
+That line isn't terribly long, but it could be if we had used longer variable
+names or more arguments to the `loop` function.
+
+We haven't covered the YS `loop` function in detail yet, but it starts with a
+set of symbol bindings and their initial values.
+Then in the tail position of the body we either `recur` with a new value for
+binding or we return the result.
+
+In YS the word `loop` and its bindings need to be on the key side and the rest
+of the body needs to be on the value side.
+So we ended up with a long line that was hard to read (even though we used
+commas to separate the bindings).
+
+It might be nicer to see each binding on a separate line.
+
+Of course we'd have to use YAML explicit keys to do that!
+
+```yaml
+    ? loop
+        i 0
+        [x y] [0 0]
+        data {}
+        [minx maxx miny maxy] [0 0 0 0]
+    : dir =: i-to-dir(i)
+      nx ny =: +
+        ...
+```
+
+YAML is often criticized for having too many features that are rarely used.
+
+That's fair, but I'd like to point out that YS has put many of those features
+to good use.
+
+YS is not just about trying to encode Clojure/Lisp forms into YAML...
+
+It's about doing that and making it actually be pretty awesome!