Add try/catch functionality (#7221)

# Description

This adds `try` (with an optional `catch` piece). Much like other
languages, `try` will try to run a block. If the block fails to run
successfully, the optional `catch` block will run if it is available.

# User-Facing Changes

This adds the `try` command.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

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

@@ -26,6 +26,7 @@ mod metadata;
 mod module;
 mod mut_;
 pub(crate) mod overlay;
+mod try_;
 mod use_;
 mod version;
 mod while_;
@@ -58,6 +59,7 @@ pub use metadata::Metadata;
 pub use module::Module;
 pub use mut_::Mut;
 pub use overlay::*;
+pub use try_::Try;
 pub use use_::Use;
 pub use version::Version;
 pub use while_::While;

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

@@ -0,0 +1,99 @@
+use nu_engine::{eval_block, CallExt};
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Block, Command, EngineState, Stack};
+use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape, Type, Value};
+
+#[derive(Clone)]
+pub struct Try;
+
+impl Command for Try {
+    fn name(&self) -> &str {
+        "try"
+    }
+
+    fn usage(&self) -> &str {
+        "Try to run a block, if it fails optionally run a catch block"
+    }
+
+    fn signature(&self) -> nu_protocol::Signature {
+        Signature::build("try")
+            .input_output_types(vec![(Type::Any, Type::Any)])
+            .required("try_block", SyntaxShape::Block, "block to run")
+            .optional(
+                "else_expression",
+                SyntaxShape::Keyword(b"catch".to_vec(), Box::new(SyntaxShape::Block)),
+                "block to run if try block fails",
+            )
+            .category(Category::Core)
+    }
+
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn is_parser_keyword(&self) -> bool {
+        true
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
+        let try_block: Block = call.req(engine_state, stack, 0)?;
+        let catch_block: Option<Block> = call.opt(engine_state, stack, 1)?;
+
+        let try_block = engine_state.get_block(try_block.block_id);
+
+        let result = eval_block(engine_state, stack, try_block, input, false, false);
+
+        match result {
+            Err(_) | Ok(PipelineData::Value(Value::Error { .. }, ..)) => {
+                if let Some(catch_block) = catch_block {
+                    let catch_block = engine_state.get_block(catch_block.block_id);
+                    eval_block(
+                        engine_state,
+                        stack,
+                        catch_block,
+                        PipelineData::new(call.head),
+                        false,
+                        false,
+                    )
+                } else {
+                    Ok(PipelineData::new(call.head))
+                }
+            }
+            Ok(output) => Ok(output),
+        }
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Try to run a missing command",
+                example: "try { asdfasdf }",
+                result: None,
+            },
+            Example {
+                description: "Try to run a missing command",
+                example: "try { asdfasdf } catch { echo 'missing' } ",
+                result: Some(Value::test_string("missing")),
+            },
+        ]
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(Try {})
+    }
+}

crates/nu-command/src/default_context.rs

@@ -60,6 +60,7 @@ pub fn create_default_context() -> EngineState {
             Metadata,
             Module,
             Mut,
+            Try,
             Use,
             Version,
             While,

crates/nu-command/tests/commands/mod.rs

@@ -82,6 +82,7 @@ mod table;
 mod take;
 mod touch;
 mod transpose;
+mod try_;
 mod uniq;
 mod update;
 mod upsert;

crates/nu-command/tests/commands/try_.rs

@@ -0,0 +1,26 @@
+use nu_test_support::nu;
+use nu_test_support::playground::Playground;
+
+#[test]
+fn try_succeed() {
+    Playground::setup("try_succeed_test", |dirs, _sandbox| {
+        let output = nu!(
+            cwd: dirs.test(),
+            "try { 345 } catch { echo 'hello' }"
+        );
+
+        assert!(output.out.contains("345"));
+    })
+}
+
+#[test]
+fn try_catch() {
+    Playground::setup("try_catch_test", |dirs, _sandbox| {
+        let output = nu!(
+            cwd: dirs.test(),
+            "try { foobarbaz } catch { echo 'hello' }"
+        );
+
+        assert!(output.out.contains("hello"));
+    })
+}