Add conditional expressions and table views

if c { a } else { b }, with else-if chains, is an expression anywhere a
value goes. Only the chosen branch runs, every branch gives the same type,
and nesting that would pass Pure's limit of 8 value-carrying joins is a
compile error.

view name = each row in list { column = value ... } declares a table with
a row per item; columns read their row as an index of the list. An index
now reads and writes its item's fields directly (at.title, at.done = 1),
so 'position' cannot name a record field.

examples/tasks.aup gains conditional views and a board table and still
matches its reference model; a new check confirms the unchosen branch is
never computed.

1 files changed, 12 insertions, 5 deletions

diff --git a/examples/tasks.aup b/examples/tasks.aup
index 4c6627f..38d06ef 100644
--- a/examples/tasks.aup
+++ b/examples/tasks.aup
@@ -27,16 +27,23 @@ on typed {
 }
 on up.down { at = at - 1 }
 on down.down { at = at + 1 }
-on toggle.down { tasks[at].done = !tasks[at].done }
+on toggle.down { at.done = !at.done }
 on drop.down { tasks.remove(at) }
 on top.down { at = tasks.first }
 on bottom.down { at = tasks.last }
 # A copy goes before the current task, which stays current.
 on again.down { tasks.insert(at, Task { title: tasks[at].title, done: tasks[at].done }) }
-on shout.down { tasks[at].title = "{tasks[at].title}!" }
+on shout.down { tasks[at].title = "{at.title}!" }
 
 view count = tasks.count
 view position = at.position
-view current = tasks[at].title
-view status = "{at.position + 1}/{tasks.count}"
-view finished = tasks[at].done
+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
+
+# 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 { " " }
+  here = task.position == at.position
+}