Add empty() function

mps-interpreter/README.md

@@ -126,6 +126,10 @@ Repeat the iterable count times, or infinite times if count is omitted.
 
 Retrieve all files from a folder, matching a regex pattern.
 
+#### empty();
+
+Empty iterator. Useful for deleting items using replacement filters.
+
 ### Sorters
 Operations to sort the items in an iterable: iterable~(sorter) OR iterable.sort(sorter)
 

mps-interpreter/src/interpretor.rs

@@ -174,5 +174,6 @@ pub(crate) fn standard_vocab(vocabulary: &mut MpsLanguageDictionary) {
         .add(crate::lang::vocabulary::repeat_function_factory())
         .add(crate::lang::vocabulary::AssignStatementFactory)
         .add(crate::lang::vocabulary::sql_init_function_factory())
-        .add(crate::lang::vocabulary::files_function_factory());
+        .add(crate::lang::vocabulary::files_function_factory())
+        .add(crate::lang::vocabulary::empty_function_factory());
 }

mps-interpreter/src/lang/vocabulary/empty.rs

@@ -0,0 +1,88 @@
+use std::collections::VecDeque;
+use std::fmt::{Debug, Display, Error, Formatter};
+use std::iter::Iterator;
+
+use crate::tokens::MpsToken;
+use crate::MpsContext;
+
+#[cfg(debug_assertions)]
+use crate::lang::utility::assert_empty;
+use crate::lang::MpsLanguageDictionary;
+use crate::lang::{MpsFunctionFactory, MpsFunctionStatementFactory, MpsIteratorItem, MpsOp};
+use crate::lang::{RuntimeError, SyntaxError};
+
+#[derive(Debug)]
+pub struct EmptyStatement {
+    context: Option<MpsContext>,
+}
+
+impl Display for EmptyStatement {
+    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
+        write!(f, "empty()")
+    }
+}
+
+impl std::clone::Clone for EmptyStatement {
+    fn clone(&self) -> Self {
+        Self {
+            context: None,
+        }
+    }
+}
+
+impl Iterator for EmptyStatement {
+    type Item = MpsIteratorItem;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        None
+    }
+}
+
+impl MpsOp for EmptyStatement {
+    fn enter(&mut self, ctx: MpsContext) {
+        self.context = Some(ctx)
+    }
+
+    fn escape(&mut self) -> MpsContext {
+        self.context.take().unwrap()
+    }
+
+    fn is_resetable(&self) -> bool {
+        true
+    }
+
+    fn reset(&mut self) -> Result<(), RuntimeError> {
+        Ok(())
+    }
+}
+
+pub struct EmptyFunctionFactory;
+
+impl MpsFunctionFactory<EmptyStatement> for EmptyFunctionFactory {
+    fn is_function(&self, name: &str) -> bool {
+        name == "empty" || name == "_"
+    }
+
+    fn build_function_params(
+        &self,
+        _name: String,
+        #[allow(unused_variables)]
+        tokens: &mut VecDeque<MpsToken>,
+        _dict: &MpsLanguageDictionary,
+    ) -> Result<EmptyStatement, SyntaxError> {
+        // empty()
+        #[cfg(debug_assertions)]
+        assert_empty(tokens)?;
+        Ok(EmptyStatement {
+            context: None,
+        })
+    }
+}
+
+pub type EmptyStatementFactory = MpsFunctionStatementFactory<EmptyStatement, EmptyFunctionFactory>;
+
+#[inline(always)]
+pub fn empty_function_factory() -> EmptyStatementFactory {
+    EmptyStatementFactory::new(EmptyFunctionFactory)
+}
+

mps-interpreter/src/lang/vocabulary/mod.rs

@@ -1,4 +1,5 @@
 mod comment;
+mod empty;
 mod files;
 mod repeat;
 mod sql_init;
@@ -7,6 +8,7 @@ mod sql_simple_query;
 mod variable_assign;
 
 pub use comment::{CommentStatement, CommentStatementFactory};
+pub use empty::{empty_function_factory, EmptyStatementFactory};
 pub use files::{files_function_factory, FilesStatementFactory};
 pub use repeat::{repeat_function_factory, RepeatStatementFactory};
 pub use sql_init::{sql_init_function_factory, SqlInitStatementFactory};

mps-interpreter/src/lib.rs

@@ -124,6 +124,10 @@
 //!
 //! Retrieve all files from a folder, matching a regex pattern.
 //!
+//! ### empty();
+//!
+//! Empty iterator. Useful for deleting items using replacement filters.
+//!
 //! ## Sorters
 //! Operations to sort the items in an iterable: iterable~(sorter) OR iterable.sort(sorter)
 //!

mps-interpreter/tests/single_line.rs

@@ -367,3 +367,12 @@ fn execute_blisssort_line() -> Result<(), Box<dyn MpsLanguageError>> {
         true,
     )
 }
+
+#[test]
+fn execute_emptyfn_line() -> Result<(), Box<dyn MpsLanguageError>> {
+    execute_single_line(
+        "empty()",
+        true,
+        true,
+    )
+}

src/help.rs

@@ -31,7 +31,10 @@ These always return an iterable which can be manipulated.
     Repeat the iterable count times, or infinite times if count is omitted.
 
  files(folder = `path/to/music`, recursive = true|false, regex = `pattern`)
-    Retrieve all files from a folder, matching a regex pattern.";
+    Retrieve all files from a folder, matching a regex pattern.
+
+ empty()
+    Empty iterator. Useful for deleting items using replacement filters.";
 
 pub const FILTERS: &str =
 "FILTERS (?filters)