Add fns and libraries

fn name(params) = value declares one expression, inlined where it is
called: arguments are computed once onto the stack and the body reads them
with PICK, so a fn costs no runtime call and keeps Pure's bound on work.
Parameters take any kind of value; recursion is a compile error.

use name reads name.aup beside the source or from AUP_PATH. A library
declares only fns and uses, sees only its parameters, and is called as
name.fn(...). Diagnostics now carry the file they are in, and the
compiler takes a loader so that sources come from the host.

examples/words.aup is such a library; examples/tasks.aup uses it and still
matches its reference model.

2 files changed, 9 insertions, 1 deletions

diff --git a/examples/tasks.aup b/examples/tasks.aup
index 38d06ef..065f649 100644
--- a/examples/tasks.aup
+++ b/examples/tasks.aup
@@ -1,6 +1,8 @@
 # A task list. The runtime owns the list: the index always names an item,
 # stays on it when items are inserted or removed before it, stops at the
 # ends, and a failed event leaves the list as it was.
+use words
+
 record Task {
   title: text
   done = 0
@@ -40,10 +42,11 @@ view position = at.position
 view current = if tasks.count > 0 { at.title } else { "(empty)" }
 view status = if tasks.count > 0 { "{at.position + 1}/{tasks.count}" } else { "empty" }
 view finished = at.done              # no value while the list is empty
+view summary = words.counted(tasks.count, "task", "tasks")
 
 # One row per task. A surface computes only the rows it shows.
 view board = each task in tasks {
   title = task.title
-  mark = if task.done { "x" } else { " " }
+  mark = words.check(task.done)
   here = task.position == at.position
 }
diff --git a/examples/words.aup b/examples/words.aup
new file mode 100644
index 0000000..2bfc0a5
--- /dev/null
+++ b/examples/words.aup
@@ -0,0 +1,5 @@
+# Words for counts and marks. A library: only fns, no state, so any app can
+# `use words` and call words.counted(...).
+fn plural(count, one, many) = if count == 1 { one } else { many }
+fn counted(count, one, many) = "{count} {plural(count, one, many)}"
+fn check(done) = if done { "x" } else { " " }