Implement delete statement for container entries

documentation/advanced-dsl.md

@@ -21,6 +21,20 @@ or `{a: 5, b: 10, c: 15}`.
 
 The `r` at the end of real literals just means that they're not cents or associated with the octave in any way. Real cent literals like `600rc` have the same size as their radical counterparts but `linear(2 * 600rc)` won't simplify to `2` like `linear(2 * 600.0)` does.
 
+## Deleting container contents
+Record or array entries can be removed using the `del` keyword. Access using individual keys, arrays of indices, boolean arrays and slices are valid targets for removal. The specified entries are popped from the array and subsequent entries are shifted back similar to how `del` works in Python.
+
+```ocaml
+"Sieve of Eratosthenes using boolean indexing with del"
+const primes = [2..100]
+let i = -1
+while (++i < length(primes))
+  del primes[primes > primes[i] vand vnot primes mod primes[i]]
+primes
+```
+
+Deleting a non-existent entry throws an error unless the access was nullish i.e. `del arr~[idx]`.
+
 ## Blocks
 Blocks start with a curly bracket `{`, have their own instance of a current scale `$` and end with `}`. The current scale is unrolled onto the parent scale at the end of the block.
 

src/ast.ts

@@ -249,6 +249,11 @@ export type ImportAllStatement = {
   module: string;
 };
 
+export type DeleteStatement = {
+  type: 'DeleteStatement';
+  entry: AccessExpression | ArraySlice;
+};
+
 export type ExpressionStatement = {
   type: 'ExpressionStatement';
   expression: Expression;
@@ -263,6 +268,7 @@ export type Statement =
   | PitchDeclaration
   | UpDeclaration
   | LiftDeclaration
+  | DeleteStatement
   | BlockStatement
   | WhileStatement
   | IfStatement

src/grammars/sonic-weave.pegjs

@@ -18,6 +18,7 @@
     'const',
     'continue',
     'defer',
+    'del',
     'dot',
     'drop',
     'ed',
@@ -71,7 +72,6 @@
     'case',
     'debugger',
     'default',
-    'delete',
     'immutable',
     'match',
     'yield',
@@ -158,6 +158,7 @@ CatchToken         = @'catch'    !IdentifierPart
 ConstToken         = @'const'    !IdentifierPart
 ContinueToken      = @'continue' !IdentifierPart
 DeferToken         = @'defer'    !IdentifierPart
+DeleteToken        = @'del'      !IdentifierPart
 DotToken           = @'dot'      !IdentifierPart
 DropToken          = @'drop'     !IdentifierPart
 EdToken            = @'ed'       !IdentifierPart
@@ -230,6 +231,7 @@ Statement
   / IterationStatement
   / TryStatement
   / DeferStatement
+  / DeleteStatement
   / ModuleDeclaration
   / ExportConstantStatement
   / ExportFunctionStatement
@@ -357,6 +359,14 @@ LiftDeclaration
     };
   }
 
+DeleteStatement
+  = DeleteToken _ entry: (TrueAccessExpression / ArraySlice) EOS {
+    return {
+      type: 'DeleteStatement',
+      entry,
+    };
+  }
+
 Parameter
   = identifier: Identifier defaultValue: (_ '=' _ @Expression)? {
     return {

src/parser/__tests__/source.spec.ts

@@ -1903,4 +1903,72 @@ describe('SonicWeave parser', () => {
       '6/3 "bob" white',
     ]);
   });
+
+  it('can delete record entries', () => {
+    const scale = expand(`{
+      const foo = {bar: 1, baz: 2}
+      del foo['bar']
+      foo
+    }`);
+    expect(scale).toEqual(['2 "baz"']);
+  });
+
+  it('can delete array elements', () => {
+    const scale = expand(`{
+      const foo = primeRange(5)
+      del foo[2]
+      foo
+    }`);
+    expect(scale).toEqual(['2', '3', '7', '11']);
+  });
+
+  it('can delete multiple array elements (boolean)', () => {
+    const scale = expand(`{
+      const primes = [2..30]
+      let i = -1
+      while (++i < length(primes))
+        del primes[primes > primes[i] vand vnot primes mod primes[i]]
+      primes
+    }`);
+    expect(scale).toEqual([
+      '2',
+      '3',
+      '5',
+      '7',
+      '11',
+      '13',
+      '17',
+      '19',
+      '23',
+      '29',
+    ]);
+  });
+
+  it('can delete multiple array elements (indices)', () => {
+    const scale = expand(`{
+      const foo = primeRange(6)
+      del foo[[1, 3]]
+      foo
+    }`);
+    expect(scale).toEqual(['2', '5', '11', '13']);
+  });
+
+  it('can delete multiple array elements (slice)', () => {
+    const scale = expand(`{
+      const foo = primeRange(7)
+      del foo[1,3..]
+      foo
+    }`);
+    expect(scale).toEqual(['2', '5', '11', '17']);
+  });
+
+  it('throws for out-of-bounds delete', () => {
+    expect(() => parseSource('del [1, 2][3]')).toThrow('Index out of range.');
+  });
+
+  it("doesn't throw for out-of-bounds nullish delete", () => {
+    expect(() => parseSource('del [1, 2]~[3]')).not.toThrow(
+      'Index out of range.'
+    );
+  });
 });

src/parser/statement.ts

@@ -43,6 +43,7 @@ import {
   ImportStatement,
   ImportAllStatement,
   MosDeclaration,
+  DeleteStatement,
 } from '../ast';
 import {
   ExpressionVisitor,
@@ -51,6 +52,7 @@ import {
   containerToArray,
 } from './expression';
 import {Tardigrade} from './mos';
+import {hasOwn} from '../utils';
 
 /**
  * An interrupt representing a return, break or continue statement.
@@ -316,12 +318,180 @@ export class StatementVisitor {
         return this.visitImportAllStatement(node);
       case 'MosDeclaration':
         return this.visitMosDeclaration(node);
+      case 'DeleteStatement':
+        return this.visitDeleteStatement(node);
       case 'EmptyStatement':
         return;
     }
     node satisfies never;
   }
 
+  protected visitDeleteStatement(node: DeleteStatement) {
+    const subVisitor = this.createExpressionVisitor();
+    const entry = node.entry;
+    const object = arrayRecordOrString(subVisitor.visit(entry.object));
+    if (typeof object === 'string') {
+      throw new Error('Strings are immutable.');
+    }
+    if (entry.type === 'AccessExpression') {
+      if (!Array.isArray(object)) {
+        const key = subVisitor.visit(entry.key);
+        if (!(typeof key === 'string')) {
+          throw new Error('Record keys must be strings.');
+        }
+        if (!hasOwn(object, key) && !entry.nullish) {
+          throw new Error(`Key error: "${key}".`);
+        }
+        delete object[key];
+        return undefined;
+      }
+      let index = subVisitor.visit(entry.key);
+      if (Array.isArray(index)) {
+        const toDelete = new Set<number>();
+        index = index.flat(Infinity);
+        for (let i = 0; i < index.length; ++i) {
+          const idx = index[i];
+          if (!(typeof idx === 'boolean' || idx instanceof Interval)) {
+            throw new Error(
+              'Only booleans and intervals can be used as indices.'
+            );
+          }
+          if (idx === true) {
+            if (i >= object.length) {
+              if (!entry.nullish) {
+                throw new Error('Indexing boolean out of range.');
+              }
+            } else {
+              toDelete.add(i);
+            }
+            continue;
+          } else if (idx === false) {
+            continue;
+          }
+          let j = idx.toInteger();
+          if (j < 0) {
+            j += object.length;
+          }
+          if (j < 0 || j >= object.length) {
+            if (!entry.nullish) {
+              throw new Error('Index out of range.');
+            }
+          } else {
+            toDelete.add(j);
+          }
+        }
+        // Delete elements from largest to smallest.
+        for (const i of Array.from(toDelete).sort((a, b) => b - a)) {
+          object.splice(i, 1);
+        }
+        return undefined;
+      }
+      if (!(index instanceof Interval)) {
+        throw new Error('Array delete access with a non-integer.');
+      }
+      let i = index.toInteger();
+      if (i < 0) {
+        i += object.length;
+      }
+      if (i < 0 || i >= object.length) {
+        if (entry.nullish) {
+          return undefined;
+        }
+        throw new Error('Index out of range.');
+      }
+      object.splice(i, 1);
+      return undefined;
+    }
+
+    entry.type satisfies 'ArraySlice';
+    if (!Array.isArray(object)) {
+      throw new Error('Array slice delete on non-array.');
+    }
+    if (
+      entry.start === null &&
+      entry.second === null &&
+      entry.penultimate === false &&
+      entry.end === null
+    ) {
+      object.length = 0;
+      return undefined;
+    }
+
+    // TODO: Refactor bounds calculation to be shared with ExpressionVisitor.visitArraySlice.
+    let start = 0;
+    let step = 1;
+    const pu = entry.penultimate;
+    let end = -1;
+
+    if (entry.start) {
+      const interval = subVisitor.visit(entry.start);
+      if (!(interval instanceof Interval)) {
+        throw new Error('Slice indices must consist of intervals.');
+      }
+      start = interval.toInteger();
+    }
+
+    if (entry.end) {
+      const interval = subVisitor.visit(entry.end);
+      if (!(interval instanceof Interval)) {
+        throw new Error('Slice indices must consist of intervals.');
+      }
+      end = interval.toInteger();
+    }
+
+    if (entry.second) {
+      const second = subVisitor.visit(entry.second);
+      if (!(second instanceof Interval)) {
+        throw new Error('Slice indices must consist of intervals.');
+      }
+      step = second.toInteger() - start;
+    }
+
+    if (start < 0) {
+      start += object.length;
+    }
+    if (end < 0) {
+      end += object.length;
+    }
+
+    const toDelete = new Set<number>();
+
+    if (step > 0) {
+      start = Math.max(0, start);
+      if ((pu ? start >= end : start > end) || start >= object.length) {
+        return undefined;
+      }
+      end = Math.min(object.length - 1, end);
+
+      toDelete.add(start);
+      let next = start + step;
+      while (pu ? next < end : next <= end) {
+        toDelete.add(next);
+        next += step;
+      }
+    } else if (step < 0) {
+      start = Math.min(object.length - 1, start);
+      if ((pu ? start <= end : start < end) || start < 0) {
+        return undefined;
+      }
+      end = Math.max(0, end);
+
+      toDelete.add(start);
+      let next = start + step;
+      while (pu ? next > end : next >= end) {
+        toDelete.add(next);
+        next += step;
+      }
+    } else {
+      throw new Error('Slice step must not be zero.');
+    }
+    // Delete elements from largest to smallest.
+    for (const i of Array.from(toDelete).sort((a, b) => b - a)) {
+      object.splice(i, 1);
+    }
+    return undefined;
+  }
+
   protected visitMosDeclaration(node: MosDeclaration) {
     if (!this.rootContext) {
       throw new Error('Root context is required.');