1 files changed, 54 insertions, 0 deletions
diff --git a/src/type/conversion.rs b/src/type/conversion.rs
index a819409..3be2874 100644
--- a/src/type/conversion.rs
+++ b/src/type/conversion.rs
@@ -71,6 +71,60 @@ impl Type {
 
     }
 
+    /// Tries to convert this type into a subtype of the other.
+    ///
+    /// ```rust
+    /// use myslip::r#type::{Type::*, util::*};
+    /// 
+    /// assert_eq!(Integer.into_type(&Boolean), Err(()));
+    /// assert_eq!(Integer.into_type(&Integer), Ok(Integer));
+    /// assert_eq!(Integer.into_type(&vt("T")), Ok(Integer));
+    /// assert_eq!(
+    ///     List(vec![Integer, Integer]).into_type(&vecof(Integer)),
+    ///     Ok(vecof(Integer))
+    /// );
+    /// assert_eq!(
+    ///     List(vec![Integer, Integer]).into_type(&vecof(vt("T"))),
+    ///     Ok(vecof(Integer))
+    /// );
+    /// assert_eq!(
+    ///     List(vec![Integer, Boolean]).into_type(&vecof(vt("T"))),
+    ///     Ok(vecof(vt("T")))
+    /// );
+    /// ```
+    pub fn into_type(self, other: &Type) -> Result<Type, ()> {
+	if !self.aka(other) {
+	    return Err(());
+	}
+
+	match (&self, other) {
+
+	    (a, b) if a == b => Ok(self),
+
+	    (_, VarType(_)) => Ok(self),
+
+	    (List(v), VecOf(b)) => match v.get(0) {
+		Some(first) => {
+		    let cand = v.into_iter()
+			.fold(first.clone(), |a, b| a.least_general_supertype(b));
+		    if cand.aka(b) {
+			Ok(vecof(cand))
+		    } else {
+			Err(())
+		    }
+		},
+		None => if NilType.aka(b) {
+		    Ok(vecof(NilType))
+		} else {
+		    Err(())
+		},
+	    },
+	    
+	    _ => Err(())
+
+	}
+    }
+
 }
 
 #[cfg(test)]