core: Implement type inference for indexing into ndarray

nac3core/src/typecheck/type_inferencer/mod.rs

@@ -1237,6 +1237,67 @@ impl<'a> Inferencer<'a> {
         Ok(boolean)
     }
 
+    /// Infers the type of a subscript expression on an `ndarray`.
+    fn infer_subscript_ndarray(
+        &mut self,
+        value: &ast::Expr<Option<Type>>,
+        dummy_tvar: Type,
+        ndims: &Type,
+    ) -> InferenceResult {
+        debug_assert!(matches!(
+            &*self.unifier.get_ty_immutable(dummy_tvar),
+            TypeEnum::TVar { is_const_generic: false, .. }
+        ));
+
+        let constrained_ty = self.unifier.add_ty(TypeEnum::TNDArray { ty: dummy_tvar, ndims: *ndims });
+        self.constrain(value.custom.unwrap(), constrained_ty, &value.location)?;
+
+        let TypeEnum::TLiteral { values, .. } = &*self.unifier.get_ty_immutable(*ndims) else {
+            panic!("Expected TLiteral for TNDArray.ndims, got {}", self.unifier.stringify(*ndims))
+        };
+
+        let ndims = values.iter()
+            .map(|ndim| match *ndim {
+                SymbolValue::U64(v) => Ok(v),
+                SymbolValue::U32(v) => Ok(v as u64),
+                SymbolValue::I32(v) => u64::try_from(v).map_err(|_| HashSet::from([
+                    format!("Expected non-negative literal for TNDArray.ndims, got {v}"),
+                ])),
+                SymbolValue::I64(v) => u64::try_from(v).map_err(|_| HashSet::from([
+                    format!("Expected non-negative literal for TNDArray.ndims, got {v}"),
+                ])),
+                _ => unreachable!(),
+            })
+            .collect::<Result<Vec<_>, _>>()?;
+
+        assert!(!ndims.is_empty());
+
+        if ndims.len() == 1 && ndims[0] == 1 {
+            // ndarray[T, Literal[1]] - Index always returns an object of type T
+
+            assert_ne!(ndims[0], 0);
+
+            Ok(dummy_tvar)
+        } else {
+            // ndarray[T, Literal[N]] where N != 1 - Index returns an object of type ndarray[T, Literal[N - 1]]
+
+            if ndims.iter().any(|v| *v == 0) {
+                unimplemented!("Inference for ndarray subscript operator with Literal[0, ...] bound unimplemented")
+            }
+
+            let ndims_min_one_ty = self.unifier.get_fresh_literal(
+                ndims.into_iter().map(|v| SymbolValue::U64(v - 1)).collect(),
+                None,
+            );
+            let subscripted_ty = self.unifier.add_ty(TypeEnum::TNDArray {
+                ty: dummy_tvar,
+                ndims: ndims_min_one_ty,
+            });
+
+            Ok(subscripted_ty)
+        }
+    }
+
     fn infer_subscript(
         &mut self,
         value: &ast::Expr<Option<Type>>,
@@ -1258,6 +1319,9 @@ impl<'a> Inferencer<'a> {
                 Ok(list_like_ty)
             }
             ExprKind::Constant { value: ast::Constant::Int(val), .. } => {
+                if let TypeEnum::TNDArray { ndims, .. } = &*self.unifier.get_ty(value.custom.unwrap()) {
+                    self.infer_subscript_ndarray(value, ty, ndims)
+                } else {
                     // the index is a constant, so value can be a sequence.
                     let ind: Option<i32> = (*val).try_into().ok();
                     let ind = ind.ok_or_else(|| HashSet::from(["Index must be int32".to_string()]))?;
@@ -1270,22 +1334,27 @@ impl<'a> Inferencer<'a> {
                     self.constrain(value.custom.unwrap(), seq, &value.location)?;
                     Ok(ty)
                 }
+            }
             _ => {
-                if let TypeEnum::TTuple { .. } = &*self.unifier.get_ty(value.custom.unwrap())
-                {
+                if let TypeEnum::TTuple { .. } = &*self.unifier.get_ty(value.custom.unwrap()) {
                     return report_error("Tuple index must be a constant (KernelInvariant is also not supported)", slice.location)
                 }
 
-                // the index is not a constant, so value can only be a list
+                // the index is not a constant, so value can only be a list-like structure
+                match &*self.unifier.get_ty(value.custom.unwrap()) {
+                    TypeEnum::TList { .. } => {
                         self.constrain(slice.custom.unwrap(), self.primitives.int32, &slice.location)?;
-                let list_like_ty = match &*self.unifier.get_ty(value.custom.unwrap()) {
-                    TypeEnum::TList { .. } => self.unifier.add_ty(TypeEnum::TList { ty }),
-                    TypeEnum::TNDArray { .. } => todo!(),
-                    _ => unreachable!(),
-                };
-                self.constrain(value.custom.unwrap(), list_like_ty, &value.location)?;
+                        let list = self.unifier.add_ty(TypeEnum::TList { ty });
+                        self.constrain(value.custom.unwrap(), list, &value.location)?;
                         Ok(ty)
                     }
+                    TypeEnum::TNDArray { ndims, .. } => {
+                        self.constrain(slice.custom.unwrap(), self.primitives.usize(), &slice.location)?;
+                        self.infer_subscript_ndarray(value, ty, ndims)
+                    }
+                    _ => unreachable!(),
+                }
+            }
         }
     }