Format implementation, fix on Echo

Now, Echo converts multiple values in a ValueStream, but it simply
forwards a single Value; if no PipelineData is detected as an input, an
empty string is returned as a single Value.

crates/nu-command/src/core_commands/echo.rs

@@ -29,10 +29,23 @@ impl Command for Echo {
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
         call.rest(engine_state, stack, 0).map(|to_be_echoed| {
-            PipelineData::Stream(ValueStream::from_stream(
-                to_be_echoed.into_iter(),
-                engine_state.ctrlc.clone(),
-            ))
+            let n = to_be_echoed.len();
+            match n.cmp(&1usize) {
+                //  More than one value is converted in a stream of values
+                std::cmp::Ordering::Greater => PipelineData::Stream(ValueStream::from_stream(
+                    to_be_echoed.into_iter(),
+                    engine_state.ctrlc.clone(),
+                )),
+
+                //  But a single value can be forwarded as it is
+                std::cmp::Ordering::Equal => PipelineData::Value(to_be_echoed[0].clone()),
+
+                //  When there are no elements, we echo the empty string
+                std::cmp::Ordering::Less => PipelineData::Value(Value::String {
+                    val: "".to_string(),
+                    span: Span::unknown(),
+                }),
+            }
         })
     }
 
@@ -41,10 +54,7 @@ impl Command for Echo {
             Example {
                 description: "Put a hello message in the pipeline",
                 example: "echo 'hello'",
-                result: Some(Value::List {
-                    vals: vec![Value::test_string("hello")],
-                    span: Span::new(0, 0),
-                }),
+                result: Some(Value::test_string("hello")),
             },
             Example {
                 description: "Print the value of the special '$nu' variable",

crates/nu-command/src/example_test.rs

@@ -24,6 +24,8 @@ pub fn test_examples(cmd: impl Command + 'static) {
         working_set.add_decl(Box::new(Math));
         working_set.add_decl(Box::new(Date));
 
+        use super::Echo;
+        working_set.add_decl(Box::new(Echo));
         // Adding the command that is being tested to the working set
         working_set.add_decl(Box::new(cmd));
 

crates/nu-command/src/strings/format/command.rs

@@ -0,0 +1,193 @@
+use nu_engine::CallExt;
+use nu_protocol::ast::{Call, PathMember};
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{
+    Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value, ValueStream,
+};
+
+#[derive(Clone)]
+pub struct Format;
+
+impl Command for Format {
+    fn name(&self) -> &str {
+        "format"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("format").required(
+            "pattern",
+            SyntaxShape::String,
+            "the pattern to output. e.g.) \"{foo}: {bar}\"",
+        )
+    }
+
+    fn usage(&self) -> &str {
+        "Format columns into a string using a simple pattern."
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let specified_pattern: Result<Value, ShellError> = call.req(engine_state, stack, 0);
+        match specified_pattern {
+            Err(e) => Err(e),
+            Ok(pattern) => {
+                //  Handle the pattern
+                let string_pattern = pattern.as_string().unwrap();
+                let ops = extract_formatting_operations(string_pattern);
+                format(input, &ops)
+            }
+        }
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Print filenames with their sizes",
+                example: "ls | format '{name}: {size}'",
+                result: None,
+            },
+            Example {
+                description: "Print elements from some columns of a table",
+                example: "echo [[col1, col2]; [v1, v2] [v3, v4]] | format '{col2}'",
+                result: Some(Value::List {
+                    vals: vec![Value::test_string("v2"), Value::test_string("v4")],
+                    span: Span::new(0, 0),
+                }),
+            },
+        ]
+    }
+}
+
+#[derive(Debug)]
+enum FormatOperation {
+    FixedText(String),
+    ValueFromColumn(String),
+}
+
+fn extract_formatting_operations(input: String) -> Vec<FormatOperation> {
+    let mut output = vec![];
+
+    let mut characters = input.chars();
+    'outer: loop {
+        let mut before_bracket = String::new();
+
+        for ch in &mut characters {
+            if ch == '{' {
+                break;
+            }
+            before_bracket.push(ch);
+        }
+
+        if !before_bracket.is_empty() {
+            output.push(FormatOperation::FixedText(before_bracket.to_string()));
+        }
+
+        let mut column_name = String::new();
+
+        for ch in &mut characters {
+            if ch == '}' {
+                break;
+            }
+            column_name.push(ch);
+        }
+
+        if !column_name.is_empty() {
+            output.push(FormatOperation::ValueFromColumn(column_name.clone()));
+        }
+
+        if before_bracket.is_empty() && column_name.is_empty() {
+            break 'outer;
+        }
+    }
+    output
+}
+
+fn format(
+    input_data: PipelineData,
+    format_operations: &[FormatOperation],
+) -> Result<PipelineData, ShellError> {
+    let data_as_value = input_data.into_value();
+    eprintln!("{:#?}", data_as_value);
+    match data_as_value {
+        Value::Record { .. } => match format_record(format_operations, &data_as_value) {
+            Ok(value) => Ok(PipelineData::Value(Value::string(value, Span::unknown()))),
+            Err(value) => Err(value),
+        },
+
+        Value::List { vals, .. } => {
+            let mut list = vec![];
+            for val in vals.iter() {
+                match val {
+                    Value::Record { .. } => match format_record(format_operations, val) {
+                        Ok(value) => {
+                            list.push(Value::string(value, Span::unknown()));
+                        }
+                        Err(value) => {
+                            return Err(value);
+                        }
+                    },
+
+                    _ => {
+                        return Err(ShellError::UnsupportedInput(
+                            "Input data is not supported by this command.".to_string(),
+                            Span::unknown(),
+                        ))
+                    }
+                }
+            }
+
+            Ok(PipelineData::Stream(ValueStream::from_stream(
+                list.into_iter(),
+                None,
+            )))
+        }
+        _ => Err(ShellError::UnsupportedInput(
+            "Input data is not supported by this command.".to_string(),
+            Span::unknown(),
+        )),
+    }
+}
+
+fn format_record(
+    format_operations: &[FormatOperation],
+    data_as_value: &Value,
+) -> Result<String, ShellError> {
+    let mut output = String::new();
+    for op in format_operations {
+        match op {
+            FormatOperation::FixedText(s) => output.push_str(s.as_str()),
+
+            //  The referenced code suggest to use the correct Span's
+            //  See: https://github.com/nushell/nushell/blob/c4af5df828135159633d4bc3070ce800518a42a2/crates/nu-command/src/commands/strings/format/command.rs#L61
+            FormatOperation::ValueFromColumn(col_name) => {
+                match data_as_value
+                    .clone()
+                    .follow_cell_path(&[PathMember::String {
+                        val: col_name.clone(),
+                        span: Span::unknown(),
+                    }]) {
+                    Ok(value_at_column) => {
+                        output.push_str(value_at_column.as_string().unwrap().as_str())
+                    }
+                    Err(se) => return Err(se),
+                }
+            }
+        }
+    }
+    Ok(output)
+}
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn test_examples() {
+        use super::Format;
+        use crate::test_examples;
+        test_examples(Format {})
+    }
+}

crates/nu-command/src/strings/format/format.rs

@@ -1,53 +0,0 @@
-//use nu_engine::CallExt;
-use nu_protocol::ast::Call;
-use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Example, PipelineData, ShellError, Signature, SyntaxShape};
-
-#[derive(Clone)]
-pub struct Format;
-
-impl Command for Format {
-    fn name(&self) -> &str {
-        "format"
-    }
-
-    fn signature(&self) -> Signature {
-        Signature::build("format").required(
-            "pattern",
-            SyntaxShape::String,
-            "the pattern to output. e.g.) \"{foo}: {bar}\"",
-        )
-    }
-
-    fn usage(&self) -> &str {
-        "Format columns into a string using a simple pattern."
-    }
-
-    fn run(
-        &self,
-        _engine_state: &EngineState,
-        _stack: &mut Stack,
-        _call: &Call,
-        _input: PipelineData,
-    ) -> Result<PipelineData, ShellError> {
-        todo!()
-    }
-
-    fn examples(&self) -> Vec<Example> {
-        vec![Example {
-            description: "Print filenames with their sizes",
-            example: "ls | format '{name}: {size}'",
-            result: None,
-        }]
-    }
-}
-
-#[cfg(test)]
-mod test {
-    #[test]
-    fn test_examples() {
-        use super::Format;
-        use crate::test_examples;
-        test_examples(Format {})
-    }
-}

crates/nu-command/src/strings/format/mod.rs

@@ -1,3 +1,3 @@
-pub mod format;
+pub mod command;
 
-pub use format::Format;
+pub use command::Format;