1 files changed, 77 insertions, 0 deletions
diff --git a/src/chomp/check/substitute.rs b/src/chomp/check/substitute.rs
new file mode 100644
index 0000000..32595b1
--- /dev/null
+++ b/src/chomp/check/substitute.rs
@@ -0,0 +1,77 @@
+use super::{
+    super::{
+        ast::{Alt, Call, Cat, Epsilon, Expression, Fix, Literal, Parameter, Variable},
+        error::SubstituteError,
+        visit::{Folder, Visitable},
+    },
+    DeepenVars, ShallowVars,
+};
+
+#[derive(Clone, Debug)]
+pub struct SubstituteParams {
+    params: Vec<Expression>,
+}
+
+impl SubstituteParams {
+    pub fn new(params: Vec<Expression>) -> Self {
+        Self { params }
+    }
+}
+
+impl Folder for SubstituteParams {
+    type Out = Result<Expression, SubstituteError>;
+
+    fn fold_epsilon(&mut self, eps: Epsilon) -> Self::Out {
+        Ok(eps.into())
+    }
+
+    fn fold_literal(&mut self, lit: Literal) -> Self::Out {
+        Ok(lit.into())
+    }
+
+    fn fold_cat(&mut self, mut cat: Cat) -> Self::Out {
+        cat.fst = Box::new(cat.fst.fold(self)?);
+        cat.snd = Box::new(cat.snd.fold(self)?);
+        Ok(cat.into())
+    }
+
+    fn fold_alt(&mut self, mut alt: Alt) -> Self::Out {
+        alt.left = Box::new(alt.left.fold(self)?);
+        alt.right = Box::new(alt.right.fold(self)?);
+        Ok(alt.into())
+    }
+
+    fn fold_fix(&mut self, mut fix: Fix) -> Self::Out {
+        for param in &mut self.params {
+            param.map(&mut DeepenVars::default());
+        }
+
+        fix.inner = Box::new(fix.inner.fold(self)?);
+
+        for param in &mut self.params {
+            param.map(&mut ShallowVars::default());
+        }
+
+        Ok(fix.into())
+    }
+
+    fn fold_variable(&mut self, var: Variable) -> Self::Out {
+        Ok(Expression::Variable(var))
+    }
+
+    fn fold_call(&mut self, mut call: Call) -> Self::Out {
+        call.args = call
+            .args
+            .into_iter()
+            .map(|arg| arg.fold(self))
+            .collect::<Result<_, _>>()?;
+        Ok(call.into())
+    }
+
+    fn fold_parameter(&mut self, param: Parameter) -> Self::Out {
+        self.params
+            .get(param.index())
+            .cloned()
+            .ok_or(SubstituteError::FreeParameter(param))
+    }
+}