add gitignore

.gitignore

@@ -0,0 +1,4 @@
+target/
+*~
+\#*
+.\#*

README.md

@@ -5,35 +5,31 @@ Rust Implementation of Ladder-Types (parsing, unification, rewriting, etc)
 
 ## Ladder Types
 
-*Ladder Types* provide a way to distinguish between multiple *concrete
-representations* of the same *abstract / conceptual type*.
+In order to implement complex datastructures and algorithms, usually
+many layers of abstraction are built ontop of each other.
+Consequently higher-level data types are encoded into lower-level data
+types, forming a chain of embeddings from concept to `rock bottom' of
+byte streams.  While a high-level type makes claims about the
+semantics of objects of that type, high-level types are ambiguous in
+regard to their concrete syntactical representation or memory
+layout. However for compositions to be type-safe, compatibility of
+concrete represenations must be ensured.
 
-The intuition is to capture a 'represented-as' relation in the
-structure of a type term. Formally, we introduce a new type
-constructor, called the *ladder type*, written as `T1 ~ T2`, where
-`T1` and `T2` are types. The term `T1 ~ T2` can be read by `T1 is
-represented as T2`.
+For example in the unix shell, many different tools & utilities
+coexist.  Depending on the application domain, each of them will
+potentially make use of different representational forms for the same
+abstract concepts. E.g. for the concept 'natural number', many
+representations do exist, e.g. with variation over radices,
+endianness, digit encoding etc.
 
-
-
-This can be particularly useful in contexts where no standard
-representational forms exist, e.g. in the unix shell where many
-different tools & utilities come into contact, each making use of
-potentially different representational forms, depending on the
-application domain.
-
-Trying to introduce a global type system at IPC level would result in
-naming conflicts: Abstract concepts like 'natural number' do not have
-any standard representational form, as it is the case with other more
-contained languages. With a simple type name like this it is not clear
-which of the many equivalent representations is required, e.g. for an
-*integer number* variations over radices, endianness, digit encoding
-etc. exist.
-
-Usually, many layers of abstraction are built ontop of each other,
-where higher-level data types are encoded into lower-level data types,
-forming a chain of embeddings from concept to `rock bottom' of byte
-streams.
+Intuitively, *ladder types* provide a way to distinguish between
+multiple *concrete representations* of the same *abstract / conceptual
+type*, by capturing the *represented-as* of layered data formats in
+the structure of type-terms. Formally, we introduce a new type
+constructor, called the *ladder type*, written `T1 ~ T2`, where `T1`
+and `T2` are types. The type-term `T1 ~ T2` then expresses the
+abstract type of `T1` being represented in terms of the concrete type
+`T2`, which can be read by "`T1` represented as `T2`".
 
 
 #### Example
@@ -42,7 +38,7 @@ each represented as unix-timestamp written as decimal number in
 big-endian, encoded as UTF-8 string.
 
 ```
-<Seq Timepoint
+<Seq TimePoint
      ~<TimeSince UnixEpoch>
      ~<Duration Seconds>
      ~ℕ
@@ -66,15 +62,18 @@ this:
 ```rust
 use laddertypes::*;
 
-let mut dict = TypeDict::new();
+fn main() {
+    let mut dict = TypeDict::new();
 
-let t1 = dict.parse("<A B~X C>").expect("couldnt parse typeterm");
-let t2 = dict.parse("<<A B~X> C>").expect("couldnt parse typeterm");
+    let t1 = dict.parse("<A B~X C>").expect("couldnt parse typeterm");
+    let t2 = dict.parse("<<A B~X> C>").expect("couldnt parse typeterm");
 
-assert_eq!( t1.clone().curry(), t2 );
-assert_eq!( t1, t2.clone().decurry() );
+    assert_eq!( t1.clone().curry(), t2 );
+    assert_eq!( t1, t2.clone().decurry() );
+}
 ```
 
-### License
+
+## License
 [GPLv3](COPYING)
 

src/lib.rs

@@ -4,9 +4,11 @@ pub mod dict;
 pub mod term;
 pub mod lexer;
 pub mod parser;
+pub mod unparser;
 pub mod curry;
 pub mod lnf;
 pub mod subtype;
+pub mod unification;
 
 #[cfg(test)]
 mod test;
@@ -14,5 +16,6 @@ mod test;
 pub use {
     dict::*,
     term::*,
+    unification::*
 };
 

src/lnf.rs

@@ -45,7 +45,7 @@ impl TypeTerm {
             }
 
             TypeTerm::App(args) => {
-                let mut args_iter = args.into_iter();
+                let args_iter = args.into_iter();
 
                 new_ladder.push( TypeTerm::App(vec![]) );
 

src/subtype.rs

@@ -19,20 +19,20 @@ impl TypeTerm {
         None
     }
 
-    pub fn is_syntactic_subtype_of(&self, expected_type: &TypeTerm) -> Result<usize, Option<(usize, usize)>> {
+    pub fn is_syntactic_subtype_of(&self, expected_type: &TypeTerm) -> Result<usize, (usize, usize)> {
         if let Some((first_match, provided_type)) = self.is_semantic_subtype_of( expected_type ) {
             let provided_lnf = provided_type.get_lnf_vec();
             let expected_lnf = expected_type.clone().get_lnf_vec();
 
             for i in 0 .. usize::min( provided_lnf.len(), expected_lnf.len() ) {
                 if provided_lnf[i] != expected_lnf[i] {
-                    return Err(Some((first_match, first_match+i)))
+                    return Err((first_match, first_match+i))
                 }
             }
 
             Ok(first_match)
         } else {
-            Err(None)
+            Err((0,0))
         }
     }
 
@@ -43,7 +43,7 @@ impl TypeTerm {
         t.is_semantic_subtype_of(self)
     }
 
-    pub fn is_syntactic_supertype_of(&self, t: &TypeTerm) -> Result<usize, Option<(usize, usize)>> {
+    pub fn is_syntactic_supertype_of(&self, t: &TypeTerm) -> Result<usize, (usize, usize)> {
         t.is_syntactic_subtype_of(self)
     }
 }

src/term.rs

@@ -78,6 +78,36 @@ impl TypeTerm {
     pub fn char_arg(&mut self, c: char) -> &mut Self {
         self.arg(TypeTerm::Char(c))
     }
+
+    /// recursively apply substitution to all subterms,
+    /// which will replace all occurences of variables which map
+    /// some type-term in `subst`
+    pub fn apply_substitution(
+        &mut self,
+        subst: &impl Fn(&TypeID) -> Option<TypeTerm>
+    ) -> &mut Self {
+        match self {
+            TypeTerm::TypeID(typid) => {
+                if let Some(t) = subst(typid) {
+                    *self = t;
+                }
+            }
+
+            TypeTerm::Ladder(rungs) => {
+                for r in rungs.iter_mut() {
+                    r.apply_substitution(subst);
+                }
+            }
+            TypeTerm::App(args) => {
+                for r in args.iter_mut() {
+                    r.apply_substitution(subst);
+                }
+            }
+            _ => {}
+        }
+
+        self
+    }
 }
 
 //<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\

src/test/mod.rs

@@ -4,4 +4,6 @@ pub mod parser;
 pub mod curry;
 pub mod lnf;
 pub mod subtype;
+pub mod substitution;
+pub mod unification;
 

src/test/parser.rs

@@ -7,9 +7,18 @@ use {
 
 #[test]
 fn test_parser_id() {
+    let mut dict = TypeDict::new();
+
+    dict.add_varname("T".into());
+
+    assert_eq!(
+        Ok(TypeTerm::TypeID(TypeID::Var(0))),
+        dict.parse("T")
+    );
+
     assert_eq!(
         Ok(TypeTerm::TypeID(TypeID::Fun(0))),
-        TypeDict::new().parse("A")
+        dict.parse("A")
     );
 }
 

src/test/substitution.rs

@@ -0,0 +1,32 @@
+
+use {
+    crate::{dict::*, term::*},
+    std::iter::FromIterator
+};
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\
+
+#[test]
+fn test_subst() {
+    let mut dict = TypeDict::new();
+
+    let mut σ = std::collections::HashMap::new();
+
+    // T  -->  ℕ
+    σ.insert
+        (dict.add_varname(String::from("T")),
+         dict.parse("ℕ").unwrap());
+
+    // U  -->  <Seq Char>
+    σ.insert
+        (dict.add_varname(String::from("U")),
+         dict.parse("<Seq Char>").unwrap());
+
+
+    assert_eq!(
+        dict.parse("<Seq T~U>").unwrap()
+            .apply_substitution(&|typid|{ σ.get(typid).cloned() }).clone(),
+        dict.parse("<Seq ℕ~<Seq Char>>").unwrap()
+    );
+}
+

src/test/subtype.rs

@@ -62,7 +62,7 @@ fn test_syntactic_subtype() {
             .is_syntactic_subtype_of(
                 &dict.parse("C~G").expect("parse errror")  
             ),
-        Err(Some((2,3)))
+        Err((2,3))
     );
 
     assert_eq!(
@@ -70,7 +70,7 @@ fn test_syntactic_subtype() {
             .is_syntactic_subtype_of(
                 &dict.parse("G~F~K").expect("parse errror")  
             ),
-        Err(None)
+        Err((0,0))
     );
 
     assert_eq!(

src/test/unification.rs

@@ -0,0 +1,118 @@
+
+use {
+    crate::{dict::*, term::*, unification::*},
+    std::iter::FromIterator
+};
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\
+
+fn test_unify(ts1: &str, ts2: &str, expect_unificator: bool) {
+    let mut dict = TypeDict::new();
+    dict.add_varname(String::from("T"));
+    dict.add_varname(String::from("U"));
+    dict.add_varname(String::from("V"));
+    dict.add_varname(String::from("W"));
+
+    let mut t1 = dict.parse(ts1).unwrap();
+    let mut t2 = dict.parse(ts2).unwrap();
+    let σ = crate::unify( &t1, &t2 );
+
+    if expect_unificator {
+        assert!(σ.is_ok());
+
+        let σ = σ.unwrap();
+
+        assert_eq!(
+            t1.apply_substitution(&|v| σ.get(v).cloned()),
+            t2.apply_substitution(&|v| σ.get(v).cloned())
+        );
+    } else {
+        assert!(! σ.is_ok());
+    }
+}
+
+#[test]
+fn test_unification_error() {
+    let mut dict = TypeDict::new();
+    dict.add_varname(String::from("T"));
+
+    assert_eq!(
+        crate::unify(
+            &dict.parse("<A T>").unwrap(),
+            &dict.parse("<B T>").unwrap()
+        ),
+
+        Err(UnificationError {
+            addr: vec![0],
+            t1: dict.parse("A").unwrap(),
+            t2: dict.parse("B").unwrap()
+        })
+    );
+
+    assert_eq!(
+        crate::unify(
+            &dict.parse("<V <U A> T>").unwrap(),
+            &dict.parse("<V <U B> T>").unwrap()
+        ),
+
+        Err(UnificationError {
+            addr: vec![1, 1],
+            t1: dict.parse("A").unwrap(),
+            t2: dict.parse("B").unwrap()
+        })
+    );
+}
+
+#[test]
+fn test_unification() {
+    test_unify("A", "A", true);
+    test_unify("A", "B", false);
+    test_unify("<Seq T>", "<Seq Ascii~Char>", true);
+    test_unify("<Seq T>", "<U Char>", true);
+
+    test_unify(
+        "<Seq Path~<Seq Char>>~<SepSeq Char '\n'>~<Seq Char>",
+        "<Seq T~<Seq Char>>~<SepSeq Char '\n'>~<Seq Char>",
+        true
+    );
+
+    let mut dict = TypeDict::new();
+
+    dict.add_varname(String::from("T"));
+    dict.add_varname(String::from("U"));
+    dict.add_varname(String::from("V"));
+    dict.add_varname(String::from("W"));
+
+    assert_eq!(
+        UnificationProblem::new(vec![
+            (dict.parse("U").unwrap(), dict.parse("<Seq Char>").unwrap()),
+            (dict.parse("T").unwrap(), dict.parse("<Seq U>").unwrap()),
+        ]).solve(),
+        Ok(
+            vec![
+                // T
+                (TypeID::Var(0), dict.parse("<Seq <Seq Char>>").unwrap()),
+
+                // U
+                (TypeID::Var(1), dict.parse("<Seq Char>").unwrap())
+            ].into_iter().collect()
+        )
+    );
+
+    assert_eq!(
+        UnificationProblem::new(vec![
+            (dict.parse("<Seq T>").unwrap(), dict.parse("<Seq W~<Seq Char>>").unwrap()),
+            (dict.parse("<Seq ℕ>").unwrap(), dict.parse("<Seq W>").unwrap()),
+        ]).solve(),
+        Ok(
+            vec![
+                // W
+                (TypeID::Var(3), dict.parse("ℕ").unwrap()),
+
+                // T
+                (TypeID::Var(0), dict.parse("ℕ~<Seq Char>").unwrap())
+            ].into_iter().collect()
+        )
+    );
+}
+

src/unification.rs

@@ -0,0 +1,100 @@
+use {
+    std::collections::HashMap,
+    crate::{term::*, dict::*}
+};
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\
+
+#[derive(Clone, Eq, PartialEq, Debug)]
+pub struct UnificationError {
+    pub addr: Vec<usize>,
+    pub t1: TypeTerm,
+    pub t2: TypeTerm
+}
+
+pub struct UnificationProblem {
+    eqs: Vec<(TypeTerm, TypeTerm, Vec<usize>)>,
+    σ: HashMap<TypeID, TypeTerm>
+}
+
+impl UnificationProblem {
+    pub fn new(eqs: Vec<(TypeTerm, TypeTerm)>) -> Self {
+        UnificationProblem {
+            eqs: eqs.iter().map(|(lhs,rhs)| (lhs.clone(),rhs.clone(),vec![])).collect(),
+            σ: HashMap::new()
+        }
+    }
+
+    pub fn eval_equation(&mut self, lhs: TypeTerm, rhs: TypeTerm, addr: Vec<usize>) -> Result<(), UnificationError> {
+        match (lhs.clone(), rhs.clone()) {
+            (TypeTerm::TypeID(TypeID::Var(varid)), t) |
+            (t, TypeTerm::TypeID(TypeID::Var(varid))) => {
+                self.σ.insert(TypeID::Var(varid), t.clone());
+
+                // update all values in substitution
+                let mut new_σ = HashMap::new();
+                for (v, tt) in self.σ.iter() {
+                    let mut tt = tt.clone();
+                    tt.apply_substitution(&|v| self.σ.get(v).cloned());
+                    new_σ.insert(v.clone(), tt);
+                }
+                self.σ = new_σ;
+
+                Ok(())
+            }
+
+            (TypeTerm::TypeID(a1), TypeTerm::TypeID(a2)) => {
+                if a1 == a2 { Ok(()) } else { Err(UnificationError{ addr, t1: lhs, t2: rhs}) }
+            }
+            (TypeTerm::Num(n1), TypeTerm::Num(n2)) => {
+                if n1 == n2 { Ok(()) } else { Err(UnificationError{ addr, t1: lhs, t2: rhs}) }
+            }
+            (TypeTerm::Char(c1), TypeTerm::Char(c2)) => {
+                if c1 == c2 { Ok(()) } else { Err(UnificationError{ addr, t1: lhs, t2: rhs}) }
+            }
+
+            (TypeTerm::Ladder(a1), TypeTerm::Ladder(a2)) |
+            (TypeTerm::App(a1), TypeTerm::App(a2)) => {
+                if a1.len() == a2.len() {
+                    for (i, (x, y)) in a1.iter().cloned().zip(a2.iter().cloned()).enumerate() {
+                        let mut new_addr = addr.clone();
+                        new_addr.push(i);
+                        self.eqs.push((x, y, new_addr));
+                    }
+                    Ok(())
+                } else {
+                    Err(UnificationError{ addr, t1: lhs, t2: rhs })
+                }
+            }
+
+            (TypeTerm::Ladder(l1), TypeTerm::Ladder(l2)) => {
+                Err(UnificationError{ addr, t1: lhs, t2: rhs })
+            }
+
+            _ => Err(UnificationError{ addr, t1: lhs, t2: rhs})
+        }
+    }
+
+    pub fn solve(mut self) -> Result<HashMap<TypeID, TypeTerm>, UnificationError> {
+        while self.eqs.len() > 0 {
+            while let Some( (mut lhs,mut rhs,addr) ) = self.eqs.pop() {
+                lhs.apply_substitution(&|v| self.σ.get(v).cloned());
+                rhs.apply_substitution(&|v| self.σ.get(v).cloned());
+                self.eval_equation(lhs, rhs, addr)?;
+            }
+        }
+
+        Ok(self.σ)
+    }
+}
+
+pub fn unify(
+    t1: &TypeTerm,
+    t2: &TypeTerm
+) -> Result<HashMap<TypeID, TypeTerm>, UnificationError> {
+    let mut unification = UnificationProblem::new(vec![ (t1.clone(), t2.clone()) ]);
+    unification.solve()
+}
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\
+

src/unparser.rs

@@ -0,0 +1,47 @@
+use crate::{dict::*, term::*};
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\
+
+impl TypeDict {
+    pub fn unparse(&self, t: &TypeTerm) -> String {
+        match t {
+            TypeTerm::TypeID(id) => self.get_typename(id).unwrap(),
+            TypeTerm::Num(n) => format!("{}", n),
+            TypeTerm::Char(c) => match c {
+                '\0' => "'\\0'".into(),
+                '\n' => "'\\n'".into(),
+                '\t' => "'\\t'".into(),
+                '\'' => "'\\''".into(),
+                c => format!("'{}'", c)
+            },
+            TypeTerm::Ladder(rungs) => {
+                let mut s = String::new();
+                let mut first = true;
+                for r in rungs.iter() {
+                    if !first {
+                        s.push('~');
+                    }
+                    first = false;
+                    s.push_str(&mut self.unparse(r));
+                }
+                s
+            }
+            TypeTerm::App(args) => {
+                let mut s = String::new();
+                s.push('<');
+                let mut first = true;
+                for r in args.iter() {
+                    if !first {
+                        s.push(' ');
+                    }
+                    first = false;
+                    s.push_str(&mut self.unparse(r));
+                }
+                s.push('>');
+                s
+            }
+        }
+    }
+}
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\