add under length

changelog.md

@@ -27,6 +27,7 @@ This version is not yet released. If you are reading this on the website, then t
 - [`under ⍜`](https://uiua.org/docs/under) [`un °`](https://uiua.org/docs/un) [`bits ⋯`](https://uiua.org/docs/bits) now preserves a minimum bit length
 - [`under ⍜`](https://uiua.org/docs/under) [`reverse ⇌`](https://uiua.org/docs/reverse) and [`under ⍜`](https://uiua.org/docs/under) [`transpose ⍉`](https://uiua.org/docs/transpose) are now smarter with respect to the ranks of [`under ⍜`](https://uiua.org/docs/under)'s second function
   - For example `⍜⇌°⊂` behaves in a more useful way
+- [`under ⍜`](https://uiua.org/docs/under) [`len ⧻`](https://uiua.org/docs/len) now reshapes an array to have the transformed length
 - `NaN` and `∞` indices for [`pick ⊡`](https://uiua.org/docs/pick) and [`select ⊏`](https://uiua.org/docs/select) now get the [`fill ⬚`](https://uiua.org/docs/fill) value
 - [`scan \\`](https://uiua.org/docs/scan) now accepts functions that take more than 2 arguments
   - The behavior is similar to the analogous behavior for [`reduce /`](https://uiua.org/docs/reduce)

site/src/primitive.rs

@@ -342,6 +342,8 @@ fn all_unders() -> impl IntoView {
             { inverse_row([Sign], No, "", "⍜±(×2) ¯5") }
             { inverse_row([First], No, "", "⍜⊢(×10) [1 2 3 4 5]") }
             { inverse_row([First, Reverse], No, "", "⍜(⊢⇌|×10) [1 2 3 4 5]") }
+            { inverse_row([Shape], No, view!(<Prim prim=Reshape/>), "⍜△⇌ [1_2_3 4_5_6]") }
+            { inverse_row([Len], No, view!(<Prim prim=Reshape/>), "⍜⧻(+1) [1_2_3 4_5_6]") }
             { inverse_row([Deshape], No, "", "⍜♭⇌ ↯3_3⇡9") }
             { inverse_row([Rise], No, "", "⍜⍏(↻¯1). [1 4 2 3 5]") }
             { inverse_row([Fall], No, "", "⍜⍖(↻¯1). [1 4 2 3 5]") }

src/algorithm/invert.rs

@@ -565,6 +565,11 @@ pub(crate) fn under_instrs(
         &pat!(Fix, (Fix), (UndoFix)),
         &pat!(UndoFix, (UndoFix), (Fix)),
         &stash1!(Shape, (Flip, Reshape)),
+        &pat!(
+            Len,
+            (CopyToUnder(1), Shape, CopyToUnder(1), First),
+            (PopUnder(1), UndoFirst, PopUnder(1), Flip, Reshape)
+        ),
         &pat!(
             Deshape,
             (Dup, Shape, PushToUnder(1), Deshape),

todo.md

@@ -2,8 +2,6 @@
 
 - 0.13
   - `base` tweak
-  - Rename and stabilize `but` and `with`
-    - Likely `with` and `off`
   - Update language tour
   - Update design page
   - Sound easter egg