From 694661d7477e4bd1e8d7ec96dcc37742d1b5ca3f Mon Sep 17 00:00:00 2001
From: tv <tv@krebsco.de>
Date: Mon, 2 Nov 2026 11:04:15 +0100
Subject: api: add type casts

---
 src/document/store.rs | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

(limited to 'src/document/store.rs')

diff --git a/src/document/store.rs b/src/document/store.rs
index 07f2e1a..9075032 100644
--- a/src/document/store.rs
+++ b/src/document/store.rs
@@ -107,6 +107,9 @@ pub(crate) enum StoreError {
     /// wanted (`Writing::retype`).
     #[error("would change {0}")]
     Retyping(Retyped),
+    /// A cast that isn't possible (`cast_path`, `typed::cast`), and why.
+    #[error("can't cast {} to {}: {why}", .from.name(), .to.name())]
+    NotCastable { from: Kind, to: Kind, why: String },
     /// A read at heads the document doesn't have (`at::At`).
     #[error("this node doesn't have these heads (yet)")]
     UnknownHeads,
@@ -1055,6 +1058,34 @@ fn counter_at(doc: &impl ReadDoc, parent: &ObjId, prop: Prop) -> Result<i64, Sto
     }
 }
 
+/// Casts what is at `path` (the newest of concurrent values, as a read resolves them) to
+/// `to` (`typed::cast`, timestamps to and from strings in `timestamps`), written as a
+/// write of the cast value would be (`write_val_path`); returns where that changed the
+/// type. Already `to`: nothing changes. Nothing there: `NotFound`; no cast possible:
+/// `NotCastable`.
+pub(crate) fn cast_path(
+    doc: &mut impl Transactable,
+    path: &[String],
+    to: Kind,
+    timestamps: TimestampFormat,
+) -> Result<Vec<Retyped>, StoreError> {
+    let held = read_held(doc, path)?.ok_or(StoreError::NotFound)?;
+    if held.kind == to {
+        return Ok(Vec::new());
+    }
+    let not_castable = |why: String| StoreError::NotCastable {
+        from: held.kind,
+        to,
+        why,
+    };
+    let value = held
+        .value
+        .as_ref()
+        .ok_or_else(|| not_castable(format!("{} is cast to nothing", held.kind.name())))?;
+    let cast = typed::cast(held.kind, value, to, timestamps).map_err(not_castable)?;
+    write_val_path(doc, path, &cast, Writing::new(false, timestamps, true))
+}
+
 /// Inserts `value` into a list: `path` ends in the index it goes to (the items from
 /// there on move up one; the list's length appends). An index past the end is
 /// `PastTheEnd`; a parent that isn't a list `NotAList`; anything but an index there, or
-- 
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