Port str startswith (#342)

Co-authored-by: Stefan Stanciulescu <contact@stefanstanciulescu.com>

crates/nu-command/src/default_context.rs

@@ -118,6 +118,7 @@ pub fn create_default_context() -> EngineState {
             StrSnakeCase,
             StrLpad,
             StrRpad,
+            StrStartsWith,
             StrReverse,
             Sys,
             Table,

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

@@ -10,6 +10,7 @@ mod length;
 mod lpad;
 mod reverse;
 mod rpad;
+mod starts_with;
 
 pub use capitalize::SubCommand as StrCapitalize;
 pub use case::*;
@@ -23,3 +24,4 @@ pub use length::SubCommand as StrLength;
 pub use lpad::SubCommand as StrLpad;
 pub use reverse::SubCommand as StrReverse;
 pub use rpad::SubCommand as StrRpad;
+pub use starts_with::SubCommand as StrStartsWith;

crates/nu-command/src/strings/str_/starts_with.rs

@@ -0,0 +1,144 @@
+use nu_engine::CallExt;
+use nu_protocol::ast::Call;
+use nu_protocol::ast::CellPath;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::Spanned;
+use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value};
+use std::sync::Arc;
+
+struct Arguments {
+    pattern: String,
+    column_paths: Vec<CellPath>,
+}
+
+#[derive(Clone)]
+
+pub struct SubCommand;
+
+impl Command for SubCommand {
+    fn name(&self) -> &str {
+        "str starts-with"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("str starts-with")
+            .required("pattern", SyntaxShape::String, "the pattern to match")
+            .rest(
+                "rest",
+                SyntaxShape::CellPath,
+                "optionally matches prefix of text by column paths",
+            )
+    }
+
+    fn usage(&self) -> &str {
+        "checks if string starts with pattern"
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        operate(engine_state, stack, call, input)
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Checks if string starts with 'my' pattern",
+                example: "'my_library.rb' | str starts-with 'my'",
+                result: Some(Value::Bool {
+                    val: true,
+                    span: Span::unknown(),
+                }),
+            },
+            Example {
+                description: "Checks if string starts with 'my' pattern",
+                example: "'Cargo.toml' | str starts-with 'Car'",
+                result: Some(Value::Bool {
+                    val: true,
+                    span: Span::unknown(),
+                }),
+            },
+            Example {
+                description: "Checks if string starts with 'my' pattern",
+                example: "'Cargo.toml' | str starts-with '.toml'",
+                result: Some(Value::Bool {
+                    val: false,
+                    span: Span::unknown(),
+                }),
+            },
+        ]
+    }
+}
+
+fn operate(
+    engine_state: &EngineState,
+    stack: &mut Stack,
+    call: &Call,
+    input: PipelineData,
+) -> Result<PipelineData, ShellError> {
+    let pattern: Spanned<String> = call.req(engine_state, stack, 0)?;
+
+    let options = Arc::new(Arguments {
+        pattern: pattern.item,
+        column_paths: call.rest(engine_state, stack, 1)?,
+    });
+    let head = call.head;
+    input.map(
+        move |v| {
+            if options.column_paths.is_empty() {
+                action(&v, &options, head)
+            } else {
+                let mut ret = v;
+                for path in &options.column_paths {
+                    let opt = options.clone();
+                    let r = ret.update_cell_path(
+                        &path.members,
+                        Box::new(move |old| action(old, &opt, head)),
+                    );
+                    if let Err(error) = r {
+                        return Value::Error { error };
+                    }
+                }
+                ret
+            }
+        },
+        engine_state.ctrlc.clone(),
+    )
+}
+
+fn action(input: &Value, Arguments { pattern, .. }: &Arguments, head: Span) -> Value {
+    match input {
+        Value::String { val: s, .. } => {
+            let starts_with = s.starts_with(pattern);
+            Value::Bool {
+                val: starts_with,
+                span: head,
+            }
+        }
+        other => Value::Error {
+            error: ShellError::UnsupportedInput(
+                format!(
+                    "Input's type is {}. This command only works with strings.",
+                    other.get_type()
+                ),
+                Span::unknown(),
+            ),
+        },
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(SubCommand {})
+    }
+}