add If operation (#8)

also

- add `value.is_truthy()`
- add `#[serde(deny_unknown_fields)]` to Operation's

rimu-value/src/lib.rs

@@ -112,6 +112,23 @@ pub fn value_get_in<'a>(value: &'a Value, keys: &[&str]) -> Option<&'a Value> {
     }
 }
 
+impl Value {
+    pub fn is_truthy(&self) -> bool {
+        match self {
+            Value::Null => false,
+            Value::Boolean(boolean) => *boolean,
+            Value::String(string) => string.len() > 0,
+            Value::Number(number) => match number {
+                Number::Unsigned(u) => u != &0_u64,
+                Number::Signed(s) => s != &0_i64,
+                Number::Float(f) => f != &0_f64,
+            },
+            Value::List(list) => list.len() > 0,
+            Value::Object(object) => object.len() > 0,
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use std::{borrow::Cow, ffi::OsString, path::PathBuf};

rimu/src/operations/eval.rs

@@ -4,6 +4,7 @@ use super::Operation;
 use crate::{Context, Engine, RenderError, Value};
 
 #[derive(Clone, Debug, PartialEq, Deserialize)]
+#[serde(deny_unknown_fields)]
 pub struct EvalOperation {
     #[serde(alias = "$eval")]
     pub expr: String,

rimu/src/operations/if_.rs

@@ -0,0 +1,80 @@
+use serde::Deserialize;
+
+use super::Operation;
+use crate::{Context, Engine, RenderError, Template, Value};
+
+#[derive(Clone, Debug, PartialEq, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct IfOperation {
+    #[serde(rename = "$if")]
+    pub condition: Box<Template>,
+    #[serde(rename = "then")]
+    pub consequent: Option<Box<Template>>,
+    #[serde(rename = "else")]
+    pub alternative: Option<Box<Template>>,
+}
+
+impl Operation for IfOperation {
+    fn render(&self, engine: &Engine, context: &Context) -> Result<Value, RenderError> {
+        let condition = engine.render(&self.condition, context)?;
+
+        let value: Value = if let Value::String(condition) = condition {
+            engine.evaluate(&condition, context)?
+        } else {
+            condition
+        };
+
+        if value.is_truthy() {
+            if let Some(consequent) = &self.consequent {
+                engine.render(consequent, context)
+            } else {
+                Ok(Value::Null)
+            }
+        } else {
+            if let Some(alternative) = &self.alternative {
+                engine.render(alternative, context)
+            } else {
+                Ok(Value::Null)
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::error::Error;
+
+    use super::*;
+    use crate::{Number, Value};
+
+    use map_macro::btree_map;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn if_() -> Result<(), Box<dyn Error>> {
+        let content = r#"
+zero:
+  $if: ten > five
+  then:
+    $eval: five
+  else:
+    $eval: ten
+"#;
+        let template: Template = serde_yaml::from_str(content)?;
+
+        let engine = Engine::default();
+        let mut context = Context::new();
+        context.insert("five", Value::Number(Number::Signed(5)));
+        context.insert("ten", Value::Number(Number::Signed(10)));
+
+        let actual: Value = engine.render(&template, &context)?;
+
+        let expected: Value = Value::Object(btree_map! {
+            "zero".into() => Value::Number(Number::Signed(5))
+        });
+
+        assert_eq!(expected, actual);
+
+        Ok(())
+    }
+}

rimu/src/operations/let_.rs

@@ -4,6 +4,7 @@ use super::Operation;
 use crate::{Context, Engine, RenderError, Template, Value};
 
 #[derive(Clone, Debug, PartialEq, Deserialize)]
+#[serde(deny_unknown_fields)]
 pub struct LetOperation {
     #[serde(rename = "$let")]
     pub variables: Box<Template>,
@@ -32,7 +33,7 @@ mod tests {
     use pretty_assertions::assert_eq;
 
     #[test]
-    fn eval() -> Result<(), Box<dyn Error>> {
+    fn let_() -> Result<(), Box<dyn Error>> {
         let content = r#"
 zero:
   $let:

rimu/src/operations/mod.rs

@@ -1,9 +1,11 @@
 mod eval;
+mod if_;
 mod let_;
 
 use serde::{de::value::MapDeserializer, Deserialize};
 
 pub use self::eval::EvalOperation;
+pub use self::if_::IfOperation;
 pub use self::let_::LetOperation;
 use crate::{Context, Engine, Object, ParseError, RenderError, Value};
 
@@ -15,13 +17,15 @@ pub trait Operation {
 pub enum Operations {
     Eval(EvalOperation),
     Let(LetOperation),
+    If(IfOperation),
 }
 
 impl Operations {
     pub(crate) fn render(&self, engine: &Engine, context: &Context) -> Result<Value, RenderError> {
         match self {
             Operations::Eval(op) => op.render(engine, context),
             Operations::Let(op) => op.render(engine, context),
+            Operations::If(op) => op.render(engine, context),
         }
     }
 }
@@ -48,6 +52,7 @@ pub(crate) fn parse_operation(operator: &str, object: &Object) -> Result<Operati
     match operator {
         "$eval" => Ok(Operations::Eval(EvalOperation::deserialize(map_de)?)),
         "$let" => Ok(Operations::Let(LetOperation::deserialize(map_de)?)),
+        "$if" => Ok(Operations::If(IfOperation::deserialize(map_de)?)),
         _ => Err(ParseError::UnknownOperator {
             operator: operator.to_owned(),
         }),

rimu/tests/spec.rs

@@ -105,3 +105,8 @@ fn interpolation() -> Result<(), Box<dyn Error>> {
 fn let_() -> Result<(), Box<dyn Error>> {
     test_specs(include_str!("./spec/let.yml"))
 }
+
+#[test]
+fn if_() -> Result<(), Box<dyn Error>> {
+    test_specs(include_str!("./spec/if.yml"))
+}

rimu/tests/spec/if.yml

@@ -0,0 +1,156 @@
+- title:    $if-then-else, true
+  context:  {cond: true}
+  template: {$if: 'cond', then: 1, else: 2}
+  output:   1
+
+- title:    $if-then-else, false
+  context:  {cond: false}
+  template: {$if: 'cond', then: 1, else: 2}
+  output:   2
+
+- title:    $if-then in array, true
+  context:  {cond: true}
+  template: [0, {$if: 'cond', then: 1}]
+  output:   [0, 1]
+
+- title:    $if-then in array, false
+  context:  {cond: false}
+  template: [0, {$if: 'cond', then: 1}]
+  output:   [0] # missing else branch should return a delete-marker
+
+- title:    $if-then in object, true
+  context:  {cond: true}
+  template: {key: {$if: 'cond', then: 1}, k2: 3}
+  output:   {key: 1, k2: 3}
+
+- title:    $if-then in object, false
+  context:  {cond: false}
+  template: {key: {$if: 'cond', then: 1}, k2: 3}
+  output:   {k2: 3} # missing else branch should return a delete-marker
+
+- title:    $if -> delete-marker, true
+  context:  {cond: true}
+  template: {key: {$if: 'cond'}, k2: 3}
+  output:   {k2: 3} # missing then/else branches should return a delete-marker
+
+- title:    $if -> delete-marker, false
+  context:  {cond: false}
+  template: {key: {$if: 'cond'}, k2: 3}
+  output:   {k2: 3} # missing then/else branches should return a delete-marker
+
+- title: $if->then, then => $eval, true
+  context: {key: {b: 1}}
+  template: {$if: 'true', then: {$eval: 'key'}}
+  output: {b: 1}
+
+- title: $if->else, else => $eval, false
+  context: {key: {b: 1}}
+  template: {$if: 'false', else: {$eval: 'key'}}
+  output: {b: 1}
+
+- title: $if->then, then => ${}, true
+  context: {key: 'one'}
+  template: {$if: 'true', then: '${key}'}
+  output: 'one'
+
+- title: $if->else, else => ${}, false
+  context: {key: 'one'}
+  template: {$if: 'false', else: '${key}'}
+  output: 'one'
+
+- title: $if->then, then => object, true
+  context: {cond: true}
+  template: {$if: 'cond', then: {key: 'hello world'}}
+  output: {key: 'hello world'}
+
+- title: $if->else, else => object, false
+  context: {cond: false}
+  template: {$if: 'cond', else: {key: 'hello world'}}
+  output: {key: 'hello world'}
+
+- title: $if->then, then => object, $eval, true
+  context: {cond: true, key: 'hello world'}
+  template: {$if: 'cond', then: {key: {$eval: 'key'}}}
+  output: {key: 'hello world'}
+
+- title: $if->else, else => object, $eval, false
+  context: {cond: false, key: 'hello world'}
+  template: {$if: 'cond', else: {key: {$eval: 'key'}}}
+  output: {key: 'hello world'}
+
+- title: $if->then, then => object, interpolation, true
+  context: {cond: true, key: 'world'}
+  template: {$if: 'cond', then: {key: 'hello ${key}'}}
+  output: {key: 'hello world'}
+
+- title: $if->else, else => object, interpolation, false
+  context: {cond: false, key: 'world'}
+  template: {$if: 'cond', else: {key: 'hello ${key}'}}
+  output: {key: 'hello world'}
+
+- title: $if->then->else, empty string
+  context: {cond: "", key: 'world'}
+  template: {$if: 'cond', then: "t", else: "f"}
+  output: f
+
+- title: $if->then->else, nonempty string
+  context: {cond: "stuff", key: 'world'}
+  template: {$if: 'cond', then: "t", else: "f"}
+  output: t
+
+- title: $if->then->else, string "0"  # once upon a time, this was false in PHP.. maybe still is
+  context: {cond: "0", key: 'world'}
+  template: {$if: 'cond', then: "t", else: "f"}
+  output: t
+
+- title: $if->then->else, zero
+  context: {cond: 0, key: 'world'}
+  template: {$if: 'cond', then: "t", else: "f"}
+  output: f
+
+- title: $if->then->else, one
+  context: {cond: 1, key: 'world'}
+  template: {$if: 'cond', then: "t", else: "f"}
+  output: t
+
+- title: $if->then->else, null
+  context: {cond: null, key: 'world'}
+  template: {$if: 'cond', then: "t", else: "f"}
+  output: f
+
+- title: $if->then->else, empty array
+  context: {cond: [], key: 'world'}
+  template: {$if: 'cond', then: "t", else: "f"}
+  output: f
+
+- title: $if->then->else, nonempty array
+  context: {cond: [1, 2], key: 'world'}
+  template: {$if: 'cond', then: "t", else: "f"}
+  output: t
+
+- title: $if->then->else, empty object
+  context: {cond: {}, key: 'world'}
+  template: {$if: 'cond', then: "t", else: "f"}
+  output: f
+
+- title: $if->then->else, nonempty object
+  context: {cond: {a: 2}, key: 'world'}
+  template: {$if: 'cond', then: "t", else: "f"}
+  output: t
+
+#- title: $if->then->else, function
+#  context: {}
+#  template: {$if: 'min', then: "t", else: "f"}
+#  output: t
+
+- title:    $if->then evaluating to nothing at the top level is null
+  context:  {cond: false}
+  template: {$if: 'cond', then: "t"}
+  output:   null
+
+- title:    $if-then-else with undefined properties
+  context:  {cond: true}
+  template: {$if: 'cond', then: 1, foo: 'bar', else: 2, bing: 'baz'}
+  error:
+    type: ParseError
+    message: 'value error: unknown field `bing`, expected one of `$if`, `then`, `else`'

rimu/tests/spec/let.yml

@@ -28,7 +28,7 @@
   context:  {}
   error:
     type: ParseError
-    message: 'value error: missing field `in`'
+    message: 'value error: unknown field `a`, expected `$let` or `in`'
 
 - title:    let array
   template: {$let: [1, 2], in: {$eval: "1 + 2"}}
@@ -70,15 +70,16 @@
 #  template: {$let: {x: [1, 2, 3]}, in: [$reverse: {$eval: "x"}]}
 #  output:   [[ 3, 2, 1 ]]
 
-#- title:    checking $let with $if without else
-#  context:  {c: 'itm'}
-#  template: {$let: {a: {$if: 'c == "item"', then: 'value'}}, in: {a: '${a}'}}
-#  error:    'InterpreterError at template.a: unknown context value a'
+- title:    checking $let with $if without else
+  context:  {c: 'itm'}
+  template: {$let: {a: {$if: 'c == "item"', then: 'value'}}, in: {a: '${a}'}}
+  error:
+    message: 'missing context: a'
 
-#- title:    let with a value of $if-then-else
-#  template: {$let: {$if: something == 3, then: {a: 10, b: 20}, else: {a: 20, b: 10}}, in: {$eval: 'a + b'}}
-#  context:  {'something': 3}
-#  output:   30
+- title:    let with a value of $if-then-else
+  template: {$let: {$if: something == 3, then: {a: 10, b: 20}, else: {a: 20, b: 10}}, in: {$eval: 'a + b'}}
+  context:  {'something': 3}
+  output:   30
 
 - title:    let with a rendered key
   template: {$let: {"first_${name}": 1, "second_${name}": 2}, in: {$eval: "first_prize + second_prize"}}