Implemented aka. Changed Nil:st type to NilType from an empty list.

4 files changed, 55 insertions, 5 deletions

diff --git a/src/type/conversion.rs b/src/type/conversion.rs
index 128d1aa..a819409 100644
--- a/src/type/conversion.rs
+++ b/src/type/conversion.rs
@@ -1,5 +1,5 @@
 
-use crate::r#type::Type;
+use crate::r#type::{Type, Type::*, util::*};
 
 impl Type {
 
@@ -27,11 +27,48 @@ impl Type {
     /// assert!(!List(vec![Integer, Boolean]).aka(&vecof(Boolean)));
     /// ```
     pub fn aka(&self, known_as: &Type) -> bool {
-	todo!()
+	self.least_general_supertype(known_as) == *known_as
     }
 
     fn least_general_supertype(&self, other: &Type) -> Type {
-	todo!()
+
+	match (self, other) {
+
+	    (a, b) if a == b => a.clone(),
+
+	    (VecOf(a), VecOf(b)) => vecof(a.least_general_supertype(b)),
+
+	    (List(v1), List(v2)) if v1.len() == v2.len() => {
+		let mut res = vec![];
+		for (a, b) in v1.into_iter().zip(v2) {
+		    res.push(a.least_general_supertype(b));
+		}
+		List(res)
+	    },
+
+	    (List(v1), List(v2)) => match v1.get(0).or(v2.get(0)) {
+		Some(first) => vecof(
+		    v1.into_iter()
+		      .chain(v2)
+		      .fold(first.clone(), |a, b| a.least_general_supertype(b))
+		),
+		None => panic!("unreachable - types would be equal")
+	    },
+
+	    (List(v), VecOf(b)) => vecof(
+		v.into_iter()
+		 .fold(*b.clone(), |a, b| a.least_general_supertype(b))
+	    ),
+	    (VecOf(b), List(v)) => vecof(
+		v.into_iter()
+		 .fold(*b.clone(), |a, b| a.least_general_supertype(b))
+	    ),
+
+
+	    _ => vt("T")
+
+	}    
+
     }
 
 }
@@ -44,8 +81,16 @@ mod tests {
     #[test]
     fn test_basic_general_supertypes() {
 
-	assert_eq!(Integer.least_general_supertype(&Integer), Integer);
-	assert_eq!(Integer.least_general_supertype(&Boolean), vt("T"));
+	assert_eq!(
+	    Integer.least_general_supertype(&Integer),
+	    Integer,
+	    "if two types are equal, they get the same supertype"
+	);
+	assert_eq!(
+	    Integer.least_general_supertype(&Boolean),
+	    vt("T"),
+	    "if two types have nothing in common, vt(\"t\") is returned"
+	);
 
     }
 
diff --git a/src/type/display.rs b/src/type/display.rs
index 4a9b54a..72e3693 100644
--- a/src/type/display.rs
+++ b/src/type/display.rs
@@ -22,6 +22,7 @@ impl fmt::Display for Type {
             QuoteTy     => write!(f, "{}", "Quote"),
             VecType     => write!(f, "{}", "Vector"),
             LetType     => write!(f, "{}", "Let"),
+	    NilType     => write!(f, "()"),
             VecOf(ty)   => write!(f, "({} ... {})", *ty, *ty),
             Arrow(a, b) => write!(f, "({} -> {})", a, b),
             List(types) => write!(
diff --git a/src/type/mod.rs b/src/type/mod.rs
index eba817b..9c2f89c 100644
--- a/src/type/mod.rs
+++ b/src/type/mod.rs
@@ -23,6 +23,8 @@ pub enum Type {
 
     List(Vec<Type>),
 
+    NilType,
+
     VecOf(Box<Type>),
     VecType, // constructor
 
@@ -108,6 +110,7 @@ impl Type {
             QuoteTy => Ok(()),
             VecType => Ok(()),
             LetType => Ok(()),
+	    NilType => Ok(()),
             Arrow(a, b) => b.is_concrete().and_then(|_ok| a.is_concrete()),
             List(v) => {
                 let mut res = Ok(());
diff --git a/src/type/subst.rs b/src/type/subst.rs
index 3ee4260..ab9d38a 100644
--- a/src/type/subst.rs
+++ b/src/type/subst.rs
@@ -44,6 +44,7 @@ impl Type {
             QuoteTy => QuoteTy,
             VecType => VecType,
             LetType => LetType,
+	    NilType => NilType,
 
         }
     }