generate: switch the position of <initial> and <closure>, so the closure can have default parameters (#13393)

# Description
Close: #12083 
Close: #12084 

# User-Facing Changes
It's a breaking change because we have switched the position of
`<initial>` and `<closure>`, after the change, initial value will be
optional. So it's possible to do something like this:

```nushell
> let f = {|fib = [0, 1]| {out: $fib.0, next: [$fib.1, ($fib.0 + $fib.1)]} }
> generate $f | first 5
╭───┬───╮
│ 0 │ 0 │
│ 1 │ 1 │
│ 2 │ 1 │
│ 3 │ 2 │
│ 4 │ 3 │
╰───┴───╯
```

It will also raise error if user don't give initial value, and the
closure don't have default parameter.
```nushell
❯ let f = {|fib| {out: $fib.0, next: [$fib.1, ($fib.0 + $fib.1)]} }
❯ generate $f
Error:   × The initial value is missing
   ╭─[entry #5:1:1]
 1 │ generate $f
   · ────┬───
   ·     ╰── Missing intial value
   ╰────
  help: Provide <initial> value in generate, or assigning default value to closure parameter

```

# Tests + Formatting
Added some test cases.

---------

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

crates/nu-command/src/generators/generate.rs

@@ -12,12 +12,12 @@ impl Command for Generate {
     fn signature(&self) -> Signature {
         Signature::build("generate")
             .input_output_types(vec![(Type::Nothing, Type::List(Box::new(Type::Any)))])
-            .required("initial", SyntaxShape::Any, "Initial value.")
             .required(
                 "closure",
                 SyntaxShape::Closure(Some(vec![SyntaxShape::Any])),
                 "Generator function.",
             )
+            .optional("initial", SyntaxShape::Any, "Initial value.")
             .allow_variants_without_examples(true)
             .category(Category::Generators)
     }
@@ -41,7 +41,7 @@ used as the next argument to the closure, otherwise generation stops.
     fn examples(&self) -> Vec<Example> {
         vec![
             Example {
-                example: "generate 0 {|i| if $i <= 10 { {out: $i, next: ($i + 2)} }}",
+                example: "generate {|i| if $i <= 10 { {out: $i, next: ($i + 2)} }} 0",
                 description: "Generate a sequence of numbers",
                 result: Some(Value::list(
                     vec![
@@ -57,10 +57,17 @@ used as the next argument to the closure, otherwise generation stops.
             },
             Example {
                 example:
-                    "generate [0, 1] {|fib| {out: $fib.0, next: [$fib.1, ($fib.0 + $fib.1)]} }",
+                    "generate {|fib| {out: $fib.0, next: [$fib.1, ($fib.0 + $fib.1)]} } [0, 1]",
                 description: "Generate a continuous stream of Fibonacci numbers",
                 result: None,
             },
+            Example {
+                example:
+                    "generate {|fib=[0, 1]| {out: $fib.0, next: [$fib.1, ($fib.0 + $fib.1)]} }",
+                description:
+                    "Generate a continuous stream of Fibonacci numbers, using default parameters",
+                result: None,
+            },
         ]
     }
 
@@ -72,15 +79,15 @@ used as the next argument to the closure, otherwise generation stops.
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
         let head = call.head;
-        let initial: Value = call.req(engine_state, stack, 0)?;
-        let closure: Closure = call.req(engine_state, stack, 1)?;
-
+        let closure: Closure = call.req(engine_state, stack, 0)?;
+        let initial: Option<Value> = call.opt(engine_state, stack, 1)?;
+        let block = engine_state.get_block(closure.block_id);
         let mut closure = ClosureEval::new(engine_state, stack, closure);
 
         // A type of Option<S> is used to represent state. Invocation
         // will stop on None. Using Option<S> allows functions to output
         // one final value before stopping.
-        let mut state = Some(initial);
+        let mut state = Some(get_initial_state(initial, &block.signature, call.head)?);
         let iter = std::iter::from_fn(move || {
             let arg = state.take()?;
 
@@ -170,6 +177,38 @@ used as the next argument to the closure, otherwise generation stops.
     }
 }
 
+fn get_initial_state(
+    initial: Option<Value>,
+    signature: &Signature,
+    span: Span,
+) -> Result<Value, ShellError> {
+    match initial {
+        Some(v) => Ok(v),
+        None => {
+            // the initial state should be referred from signature
+            if !signature.optional_positional.is_empty()
+                && signature.optional_positional[0].default_value.is_some()
+            {
+                Ok(signature.optional_positional[0]
+                    .default_value
+                    .clone()
+                    .expect("Already checked default value"))
+            } else {
+                Err(ShellError::GenericError {
+                    error: "The initial value is missing".to_string(),
+                    msg: "Missing initial value".to_string(),
+                    span: Some(span),
+                    help: Some(
+                        "Provide an <initial> value as an argument to generate, or assign a default value to the closure parameter"
+                            .to_string(),
+                    ),
+                    inner: vec![],
+                })
+            }
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;