nu_cmd_lang/core_commands/
if_.rs

1use itertools::Itertools;
2use nu_engine::command_prelude::*;
3use nu_protocol::{
4    engine::{CommandType, StateWorkingSet},
5    eval_const::{eval_const_subexpression, eval_constant, eval_constant_with_input},
6};
7
8#[derive(Clone)]
9pub struct If;
10
11impl Command for If {
12    fn name(&self) -> &str {
13        "if"
14    }
15
16    fn description(&self) -> &str {
17        "Conditionally run a block."
18    }
19
20    fn signature(&self) -> nu_protocol::Signature {
21        Signature::build("if")
22            .input_output_types(vec![(Type::Any, Type::Any)])
23            .required("cond", SyntaxShape::MathExpression, "Condition to check.")
24            .required(
25                "then_block",
26                SyntaxShape::Block,
27                "Block to run if check succeeds.",
28            )
29            .optional(
30                "else_expression",
31                SyntaxShape::Keyword(
32                    b"else".to_vec(),
33                    Box::new(SyntaxShape::OneOf(vec![
34                        SyntaxShape::Block,
35                        SyntaxShape::Expression,
36                    ])),
37                ),
38                "Expression or block to run when the condition is false.",
39            )
40            .category(Category::Core)
41    }
42
43    fn extra_description(&self) -> &str {
44        "This command is a parser keyword. For details, check:
45  https://www.nushell.sh/book/thinking_in_nu.html"
46    }
47
48    fn command_type(&self) -> CommandType {
49        CommandType::Keyword
50    }
51
52    fn is_const(&self) -> bool {
53        true
54    }
55
56    fn run_const(
57        &self,
58        working_set: &StateWorkingSet,
59        call: &Call,
60        input: PipelineData,
61    ) -> Result<PipelineData, ShellError> {
62        let call = call.assert_ast_call()?;
63
64        let ([cond, then_expr], else_case) = {
65            let mut iter = call.positional_iter();
66            (
67                iter.next_array().expect("checked through parser"),
68                iter.next(),
69            )
70        };
71
72        let then_block = then_expr
73            .as_block()
74            .ok_or_else(|| ShellError::TypeMismatch {
75                err_message: "expected block".into(),
76                span: then_expr.span,
77            })?;
78
79        if eval_constant(working_set, cond)?.as_bool()? {
80            let block = working_set.get_block(then_block);
81            eval_const_subexpression(working_set, block, input, block.span.unwrap_or(call.head))
82        } else if let Some(else_case) = else_case {
83            if let Some(else_expr) = else_case.as_keyword() {
84                if let Some(block_id) = else_expr.as_block() {
85                    let block = working_set.get_block(block_id);
86                    eval_const_subexpression(
87                        working_set,
88                        block,
89                        input,
90                        block.span.unwrap_or(call.head),
91                    )
92                } else {
93                    eval_constant_with_input(working_set, else_expr, input)
94                }
95            } else {
96                eval_constant_with_input(working_set, else_case, input)
97            }
98        } else {
99            Ok(PipelineData::empty())
100        }
101    }
102
103    fn run(
104        &self,
105        _engine_state: &EngineState,
106        _stack: &mut Stack,
107        _call: &Call,
108        _input: PipelineData,
109    ) -> Result<PipelineData, ShellError> {
110        // This is compiled specially by the IR compiler. The code here is never used when
111        // running in IR mode.
112        eprintln!(
113            "Tried to execute 'run' for the 'if' command: this code path should never be reached in IR mode"
114        );
115        unreachable!()
116    }
117
118    fn search_terms(&self) -> Vec<&str> {
119        vec!["else", "conditional"]
120    }
121
122    fn examples(&self) -> Vec<Example<'_>> {
123        vec![
124            Example {
125                description: "Output a value if a condition matches, otherwise return nothing.",
126                example: "if 2 < 3 { 'yes!' }",
127                result: Some(Value::test_string("yes!")),
128            },
129            Example {
130                description: "Output a value if a condition matches, else return another value.",
131                example: "if 5 < 3 { 'yes!' } else { 'no!' }",
132                result: Some(Value::test_string("no!")),
133            },
134            Example {
135                description: "Chain multiple if's together.",
136                example: "if 5 < 3 { 'yes!' } else if 4 < 5 { 'no!' } else { 'okay!' }",
137                result: Some(Value::test_string("no!")),
138            },
139        ]
140    }
141}
142
143#[cfg(test)]
144mod test {
145    use super::*;
146
147    #[test]
148    fn test_examples() -> nu_test_support::Result {
149        nu_test_support::test().examples(If)
150    }
151}
nu_cmd_lang/core_commands/if_.rs

nu_cmd_lang/core_commands/
if_.rs
