Type Annotations

- Add type annotations: [type] statement
- Add type definitions: [[name] type], [[name] := statement]
- Add type annotations example (08)
- add Quickstart.md, reference it from README

Quickstart.md

@@ -0,0 +1,47 @@
+# Hello, World!
+
+```
+"Hello, World!".println
+```
+This calls the `println` function with the argument
+`"Hello, World!"` (a string).
+
+`println` then prints the argument (to stdout),
+causing it to show up in your terminal.
+
+# Hello, Variable!
+
+To declare a variable in mers, use `:=`:
+
+```
+greeting := "Hello, World!"
+```
+
+To retrieve its value, simply write its name.
+
+```
+greeting.println
+```
+
+# Conditions
+
+(todo)
+
+# Functions
+
+Functions represent actions. Many represent transformations from some input to some output.
+They are written as `argument -> action`:
+
+```
+a -> if a false else true
+```
+
+# Types
+
+(todo)
+
+## Type Annotations
+
+## Custom Types
+
+### Defining Custom Types

README.md

@@ -3,6 +3,9 @@
 Mers is a high-level programming language.
 It is designed to be safe (it doesn't crash at runtime) and as simple as possible.
 
+See also:
+[Quickstart](Quickstart.md),
+
 ## what makes it special
 
 ### Simplicity

examples/08_Type_Annotations.mers

@@ -0,0 +1,76 @@
+// Change a statement's output type:
+// `[type] statement`
+
+my_list := [List<Int/Float>] ().as_list
+
+&my_list.push(12)
+&my_list.push(0.5)
+
+my_list.println
+
+
+// This can also be used to cause a compiler error on invalid types:
+// `[type] statement`
+// If `statement` isn't included in `type`, this will cause an error.
+
+my_list.for_each(v ->
+  v.try((
+    v -> {
+      [Int] v // this won't compile if v isn't a float
+      ("Int: ", v).concat
+    }
+    v -> {
+      [Float] v
+      ("Float: ", v).concat
+    }
+  )).println
+)
+
+
+// These annotations work on all kinds of statements
+
+// Function that always returns Int/Float
+double := x -> [Int/Float] {
+  (x, x).sum
+}
+
+// Function that must take Int/Float, but,
+// if given Int, returns Int, and if given Float, returns Float.
+// Therefore usually nicer than the other `double`
+double := x -> {
+  [Int/Float] x
+  (x, x).sum
+}
+
+
+// Define custom types:
+// `[[MyType] TypeDefinition]`
+
+[[City] (Int, Int, Int)]
+
+get_coords := city -> [(Int, Int)] {
+  // only works with values of type City
+  (_, x, y) := [City] city
+  // return the coords
+  (x, y)
+}
+
+test_city := (56000, 127, -12)
+
+("Coords: ", test_city.get_coords).concat.println
+
+
+// Define custom types that depend on the environment:
+// `[[MyType] := some_statement]`
+// Note: `some_statement` will be compiled and checked, but never executed, since we only care about its type at compile-time, not its actual value at runtime.
+
+make_list_maybe_none := (val_1, val_2) -> {
+  // ListType is List<T> where T is the type of val_1/val_2/()
+  [[ListType] := (val_1, val_2, ()).as_list]
+  // return a list with that type
+  [ListType] (val_1, val_2).as_list
+}
+
+maybe_none_list := ("one", 2).make_list_maybe_none
+maybe_none_list.println
+maybe_none_list // use `--check only` to see the type: `List<String/Int/()>`

mers_lib/src/data/mod.rs

@@ -374,16 +374,16 @@ impl Display for Type {
         if self.types.is_empty() {
             write!(f, "<unreachable>")
         } else {
-            if self.types.len() > 1 {
+            // if self.types.len() > 1 {
-                write!(f, "{{")?;
+            //     write!(f, "{{")?;
-            }
+            // }
             write!(f, "{}", self.types[0])?;
             for t in self.types.iter().skip(1) {
                 write!(f, "/{t}")?;
             }
-            if self.types.len() > 1 {
+            // if self.types.len() > 1 {
-                write!(f, "}}")?;
+            //     write!(f, "}}")?;
-            }
+            // }
             Ok(())
         }
     }

mers_lib/src/data/reference.rs

@@ -74,6 +74,10 @@ impl Display for Reference {
 }
 impl Display for ReferenceT {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "&{}", self.0)
+        if self.0.types.len() > 1 {
+            write!(f, "&{{{}}}", self.0)
+        } else {
+            write!(f, "&{}", self.0)
+        }
     }
 }

mers_lib/src/errors/mod.rs

@@ -63,6 +63,13 @@ pub mod error_colors {
     pub const AssignFrom: Color = InitFrom;
     pub const AssignTo: Color = InitTo;
     pub const AssignTargetNonReference: Color = Color::BrightYellow;
+
+    pub const AsTypeStatementWithTooBroadType: Color = InitFrom;
+    pub const AsTypeTypeAnnotation: Color = InitTo;
+
+    pub const BadCharInTupleType: Color = Color::Red;
+    pub const BadTypeFromParsed: Color = Color::Blue;
+    pub const TypeAnnotationNoClosingBracket: Color = Color::Blue;
 }
 enum CheckErrorComponent {
     Message(String),

mers_lib/src/info/mod.rs

@@ -3,26 +3,27 @@ use std::fmt::Debug;
 #[derive(Clone, Debug)]
 pub struct Info<L: Local> {
     pub scopes: Vec<L>,
+    pub global: L::Global,
 }
 
 impl<L: Local> Info<L> {
     /// Returns self, but completely empty (even without globals).
     /// Only use this if you assume this Info will never be used.
     pub fn neverused() -> Self {
-        Self { scopes: vec![] }
+        Self {
+            scopes: vec![],
+            global: L::Global::default(),
+        }
     }
 }
 
 pub trait Local: Default + Debug {
     type VariableIdentifier;
     type VariableData;
-    // type TypesIdentifier;
+    type Global: Default + Debug + Clone;
-    // type TypesType;
     fn init_var(&mut self, id: Self::VariableIdentifier, value: Self::VariableData);
     fn get_var(&self, id: &Self::VariableIdentifier) -> Option<&Self::VariableData>;
     fn get_var_mut(&mut self, id: &Self::VariableIdentifier) -> Option<&mut Self::VariableData>;
-    // fn add_type(&mut self, id: Self::TypesIdentifier, new_type: Self::TypesType);
-    // fn get_type(&self, id: Self::TypesIdentifier) -> Option<&Self::TypesType>;
     fn duplicate(&self) -> Self;
 }
 
@@ -39,6 +40,7 @@ impl<L: Local> Info<L> {
 impl<L: Local> Local for Info<L> {
     type VariableIdentifier = L::VariableIdentifier;
     type VariableData = L::VariableData;
+    type Global = ();
     fn init_var(&mut self, id: Self::VariableIdentifier, value: Self::VariableData) {
         self.scopes.last_mut().unwrap().init_var(id, value)
     }
@@ -51,6 +53,7 @@ impl<L: Local> Local for Info<L> {
     fn duplicate(&self) -> Self {
         Self {
             scopes: self.scopes.iter().map(|v| v.duplicate()).collect(),
+            global: self.global.clone(),
         }
     }
 }
@@ -59,6 +62,7 @@ impl<L: Local> Default for Info<L> {
     fn default() -> Self {
         Self {
             scopes: vec![L::default()],
+            global: L::Global::default(),
         }
     }
 }

mers_lib/src/parsing/mod.rs

@@ -56,6 +56,17 @@ impl Source {
         let content = std::fs::read_to_string(&path)?;
         Ok(Self::new(SourceFrom::File(path), content))
     }
+    pub fn new_from_string_raw(source: String) -> Self {
+        Self {
+            src_from: SourceFrom::Unspecified,
+            src_raw_len: source.len(),
+            src_og: source.clone(),
+            src: source,
+            comments: vec![],
+            i: 0,
+            sections: vec![],
+        }
+    }
     pub fn new_from_string(source: String) -> Self {
         Self::new(SourceFrom::Unspecified, source)
     }
@@ -167,7 +178,7 @@ impl Source {
         Some(ch)
     }
     fn word_splitter(ch: char) -> bool {
-        ch.is_whitespace() || ".,;[](){}".contains(ch)
+        ch.is_whitespace() || ".,;[](){}/<".contains(ch)
     }
     pub fn peek_word(&self) -> &str {
         self.src[self.i..]

mers_lib/src/parsing/statements.rs

@@ -4,13 +4,107 @@ use super::{Source, SourcePos};
 use crate::{
     data::Data,
     errors::{error_colors, CheckError},
-    program::{self, parsed::MersStatement},
+    program::{
+        self,
+        parsed::{as_type::AsType, MersStatement},
+    },
 };
 
 pub fn parse(
     src: &mut Source,
 ) -> Result<Option<Box<dyn program::parsed::MersStatement>>, CheckError> {
     src.section_begin("statement".to_string());
+    src.skip_whitespace();
+    // type annotation:
+    //  [type] statement // force output type to be `type`
+    //  [[name] type] // define `name` as `type`
+    //  [[name] := statement] // define `name` as the type of `statement` (`statement` is never executed)
+    if matches!(src.peek_char(), Some('[')) {
+        let pos_in_src = src.get_pos();
+        src.next_char();
+        return Ok(Some(if matches!(src.peek_char(), Some('[')) {
+            src.next_char();
+            // [[...
+            let name = src.next_word();
+            let name = name.trim().to_owned();
+            src.skip_whitespace();
+            if !matches!(src.next_char(), Some(']')) {
+                return Err(
+                    CheckError::new().msg(format!("Expected ']' after type name in [[type_name]]"))
+                );
+            }
+            src.skip_whitespace();
+            if src.peek_word() == ":=" {
+                src.next_word();
+                // [[name] := statement]
+                let statement = match parse(src) {
+                    Ok(Some(v)) => v,
+                    Ok(None) => {
+                        return Err(CheckError::new()
+                            .src(vec![((pos_in_src, src.get_pos()).into(), None)])
+                            .msg(format!("EOF after `[...]` type annotation")))
+                    }
+                    Err(e) => return Err(e),
+                };
+                if !matches!(src.next_char(), Some(']')) {
+                    return Err(CheckError::new().msg(format!(
+                        "Expected ']' after statement in [[type_name] := statement]"
+                    )));
+                }
+                Box::new(program::parsed::custom_type::CustomType {
+                    pos_in_src: (pos_in_src, src.get_pos()).into(),
+                    name,
+                    source: Err(statement),
+                })
+            } else {
+                // [[name] type]
+                src.skip_whitespace();
+                let as_type = super::types::parse_type(src)?;
+                src.skip_whitespace();
+                if !matches!(src.next_char(), Some(']')) {
+                    return Err(CheckError::new().msg(format!(
+                        "Expected ']' after type definition in [[type_name] type_definition]"
+                    )));
+                }
+                Box::new(program::parsed::custom_type::CustomType {
+                    pos_in_src: (pos_in_src, src.get_pos()).into(),
+                    name,
+                    source: Ok(as_type),
+                })
+            }
+        } else {
+            // [type] statement
+            src.skip_whitespace();
+            let type_pos_in_src = src.get_pos();
+            let as_type = super::types::parse_type(src)?;
+            let type_pos_in_src = (type_pos_in_src, src.get_pos()).into();
+            src.skip_whitespace();
+            if !matches!(src.next_char(), Some(']')) {
+                return Err(CheckError::new()
+                    .src(vec![(
+                        (pos_in_src, src.get_pos()).into(),
+                        Some(error_colors::TypeAnnotationNoClosingBracket),
+                    )])
+                    .msg(format!("Missing closing bracket ']' after type annotation")));
+            }
+            let statement = match parse(src) {
+                Ok(Some(v)) => v,
+                Ok(None) => {
+                    return Err(CheckError::new()
+                        .src(vec![((pos_in_src, src.get_pos()).into(), None)])
+                        .msg(format!("EOF after `[...]` type annotation")))
+                }
+                Err(e) => return Err(e),
+            };
+            Box::new(AsType {
+                pos_in_src: (pos_in_src, src.get_pos()).into(),
+                statement,
+                as_type,
+                type_pos_in_src,
+                expand_type: true,
+            })
+        }));
+    }
     let mut first = if let Some(s) = parse_no_chain(src)? {
         s
     } else {
@@ -128,8 +222,8 @@ pub fn parse_multiple(
 pub fn parse_no_chain(
     src: &mut Source,
 ) -> Result<Option<Box<dyn program::parsed::MersStatement>>, CheckError> {
-    src.section_begin("statement no chain".to_string());
     src.skip_whitespace();
+    src.section_begin("statement no chain".to_string());
     match src.peek_char() {
         Some('#') => {
             let pos_in_src = src.get_pos();
@@ -339,6 +433,14 @@ pub fn parse_no_chain(
 
 /// expects to be called *after* a " character is consumed from src
 pub fn parse_string(src: &mut Source, double_quote: SourcePos) -> Result<String, CheckError> {
+    parse_string_custom_end(src, double_quote, '"', '"')
+}
+pub fn parse_string_custom_end(
+    src: &mut Source,
+    opening: SourcePos,
+    opening_char: char,
+    closing_char: char,
+) -> Result<String, CheckError> {
     let mut s = String::new();
     loop {
         if let Some(ch) = src.next_char() {
@@ -350,6 +452,7 @@ pub fn parse_string(src: &mut Source, double_quote: SourcePos) -> Result<String,
                     Some('n') => '\n',
                     Some('t') => '\t',
                     Some('"') => '"',
+                    Some(c) if c == closing_char || c == opening_char => c,
                     Some(o) => {
                         return Err(CheckError::new()
                             .src(vec![(
@@ -367,7 +470,7 @@ pub fn parse_string(src: &mut Source, double_quote: SourcePos) -> Result<String,
                             .msg(format!("EOF in backslash escape")));
                     }
                 });
-            } else if ch == '"' {
+            } else if ch == closing_char {
                 break;
             } else {
                 s.push(ch);
@@ -375,11 +478,27 @@ pub fn parse_string(src: &mut Source, double_quote: SourcePos) -> Result<String,
         } else {
             return Err(CheckError::new()
                 .src(vec![(
-                    (double_quote, src.get_pos()).into(),
+                    (opening, src.get_pos()).into(),
                     Some(error_colors::StringEOF),
                 )])
-                .msg(format!("EOF in string literal")));
+                .msg(format!(
+                    "EOF in string literal{}",
+                    if closing_char != '"' {
+                        format!(
+                            "{opening_char}...{closing_char} (end string with '{closing_char}')"
+                        )
+                    } else {
+                        String::new()
+                    }
+                )));
         }
     }
     Ok(s)
 }
+pub fn to_string_literal(val: &str, end: char) -> String {
+    val.replace("\\", "\\\\")
+        .replace("\r", "\\r")
+        .replace("\n", "\\n")
+        .replace("\"", "\\\"")
+        .replace(end, format!("\\{end}").as_str())
+}

mers_lib/src/parsing/types.rs

@@ -1,47 +1,113 @@
+use std::sync::Arc;
+
+use crate::{
+    data::{self, Type},
+    errors::{error_colors, CheckError},
+};
+
 use super::Source;
 
 /// multiple types are represented as a `Vec<ParsedType>`.
+#[derive(Clone, Debug)]
 pub enum ParsedType {
+    Reference(Vec<Self>),
     Tuple(Vec<Vec<Self>>),
     Type(String),
+    TypeWithInfo(String, String),
 }
 
-fn parse_single_type(src: &mut Source) -> Result<ParsedType, ()> {
+pub fn parse_single_type(src: &mut Source) -> Result<ParsedType, CheckError> {
     src.section_begin("parse single type".to_string());
     src.skip_whitespace();
     Ok(match src.peek_char() {
+        // Reference
+        Some('&') => {
+            src.next_char();
+            if let Some('{') = src.peek_char() {
+                src.next_char();
+                let types = parse_type(src)?;
+                let nc = src.next_char();
+                if !matches!(nc, Some('}')) {
+                    let nc = if let Some(nc) = nc {
+                        format!("'{nc}'")
+                    } else {
+                        format!("EOF")
+                    };
+                    return Err(CheckError::new().msg(format!(
+                        "No closing }} in reference type with opening {{! Found {nc} instead"
+                    )));
+                }
+                ParsedType::Reference(types)
+            } else {
+                ParsedType::Reference(vec![parse_single_type(src)?])
+            }
+        }
         // Tuple
         Some('(') => {
+            let pos_in_src = src.get_pos();
             src.next_char();
             src.section_begin("parse tuple's inner types".to_string());
             let mut inner = vec![];
-            loop {
+            src.skip_whitespace();
-                match src.peek_char() {
+            if let Some(')') = src.peek_char() {
-                    Some(')') => {
+                // empty tuple, don't even start the loop
-                        src.next_char();
+            } else {
-                        break;
+                loop {
+                    inner.push(parse_type(src)?);
+                    match src.peek_char() {
+                        Some(')') => {
+                            src.next_char();
+                            break;
+                        }
+                        Some(',') => {
+                            src.next_char();
+                        }
+                        _ => {
+                            let ppos = src.get_pos();
+                            src.next_char();
+                            return Err(CheckError::new()
+                                .src(vec![
+                                    ((pos_in_src, src.get_pos()).into(), None),
+                                    (
+                                        (ppos, src.get_pos()).into(),
+                                        Some(error_colors::BadCharInTupleType),
+                                    ),
+                                ])
+                                .msg(format!(
+                                "Unexpected character in tuple type, expected comma ',' or ')'."
+                            )));
+                        }
                     }
-                    Some(',') => {
-                        src.next_char();
-                    }
-                    _ => todo!("err: bad char in tuple inner type"),
                 }
-                inner.push(parse_type(src)?);
             }
             ParsedType::Tuple(inner)
         }
-        Some(_) => ParsedType::Type(src.next_word().to_lowercase()),
+        Some(_) => {
+            let t = src.next_word().to_owned();
+            src.skip_whitespace();
+            if let Some('<') = src.peek_char() {
+                let pos = src.get_pos();
+                src.next_char();
+                ParsedType::TypeWithInfo(
+                    t,
+                    super::statements::parse_string_custom_end(src, pos, '<', '>')?,
+                )
+            } else {
+                ParsedType::Type(t)
+            }
+        }
         None => todo!(),
     })
 }
 
-fn parse_type(src: &mut Source) -> Result<Vec<ParsedType>, ()> {
+pub fn parse_type(src: &mut Source) -> Result<Vec<ParsedType>, CheckError> {
     src.section_begin("parse single type".to_string());
     let mut types = vec![];
     loop {
         types.push(parse_single_type(src)?);
         src.skip_whitespace();
         if let Some('/') = src.peek_char() {
+            src.next_char();
             continue;
         } else {
             break;
@@ -49,3 +115,49 @@ fn parse_type(src: &mut Source) -> Result<Vec<ParsedType>, ()> {
     }
     Ok(types)
 }
+pub fn type_from_parsed(
+    parsed: &Vec<ParsedType>,
+    info: &crate::program::run::CheckInfo,
+) -> Result<Type, CheckError> {
+    let mut as_type = Type::empty();
+    for t in parsed.iter() {
+        as_type.add(match t {
+            ParsedType::Reference(inner) => {
+                let inner = type_from_parsed(inner, info)?;
+                Arc::new(data::reference::ReferenceT(inner))
+            }
+            ParsedType::Tuple(t) => Arc::new(data::tuple::TupleT(
+                t.iter()
+                    .map(|v| type_from_parsed(v, info))
+                    .collect::<Result<_, _>>()?,
+            )),
+            ParsedType::Type(name) => match info
+                .scopes
+                .iter()
+                .find_map(|scope| scope.types.iter().find(|v| v.0 == name).map(|(_, v)| v))
+            {
+                Some(Ok(t)) => Arc::clone(t),
+                Some(Err(_)) => {
+                    return Err(CheckError::new().msg(format!(
+                        "Type: specified type without info, but type needs additional info"
+                    )))
+                }
+                None => return Err(CheckError::new().msg(format!("Unknown type '{name}'"))),
+            },
+            ParsedType::TypeWithInfo(name, additional_info) => match info
+                .scopes
+                .iter()
+                .find_map(|scope| scope.types.iter().find(|v| v.0 == name).map(|(_, v)| v))
+            {
+                Some(Ok(t)) => {
+                    return Err(CheckError::new().msg(format!(
+                        "Type: specified type with info, but type {t} doesn't need it"
+                    )))
+                }
+                Some(Err(f)) => f(&additional_info, info)?,
+                None => return Err(CheckError::new().msg(format!("Unknown type '{name}'"))),
+            },
+        });
+    }
+    Ok(as_type)
+}

mers_lib/src/program/configs/mod.rs

@@ -1,8 +1,10 @@
 use std::sync::{Arc, RwLock};
 
 use crate::{
-    data::{Data, Type},
+    data::{self, Data, MersType},
+    errors::CheckError,
     info::Local,
+    program::run::CheckInfo,
 };
 
 mod with_base;
@@ -58,11 +60,27 @@ impl Config {
     }
 
     pub fn new() -> Self {
+        let mut info_check: CheckInfo = Default::default();
+        macro_rules! init_d {
+            ($e:expr) => {
+                let t = $e;
+                info_check
+                    .scopes
+                    .last_mut()
+                    .unwrap()
+                    .types
+                    .insert(t.to_string(), Ok(Arc::new(t)));
+            };
+        }
+        init_d!(data::bool::BoolT);
+        init_d!(data::int::IntT);
+        init_d!(data::float::FloatT);
+        init_d!(data::string::StringT);
         Self {
             globals: 0,
             info_parsed: Default::default(),
             info_run: Default::default(),
-            info_check: Default::default(),
+            info_check,
         }
     }
 
@@ -74,8 +92,15 @@ impl Config {
         self.globals += 1;
         self
     }
-    pub fn add_type(self, _name: String, _t: Type) -> Self {
+    pub fn add_type(
-        // TODO! needed for type syntax in the parser, everything else probably(?) works already
+        mut self,
+        name: String,
+        t: Result<
+            Arc<dyn MersType>,
+            Arc<dyn Fn(&str, &CheckInfo) -> Result<Arc<dyn MersType>, CheckError> + Send + Sync>,
+        >,
+    ) -> Self {
+        self.info_check.scopes[0].types.insert(name, t);
         self
     }
 

mers_lib/src/program/configs/with_list.rs

@@ -5,6 +5,7 @@ use std::{
 
 use crate::{
     data::{self, Data, MersData, MersType, Type},
+    parsing::{statements::to_string_literal, Source},
     program::{self, run::CheckInfo},
 };
 
@@ -20,7 +21,10 @@ impl Config {
     /// `get_mut: fn` like get, but returns a reference to the object
     pub fn with_list(self) -> Self {
         // TODO: Type with generics
-        self.add_type("List".to_string(), Type::new(ListT(Type::empty_tuple())))
+        self.add_type("List".to_string(),
+            Err(Arc::new(|s, i| {
+                let t = crate::parsing::types::parse_type(&mut Source::new_from_string_raw(s.to_owned()))?;
+                Ok(Arc::new(ListT(crate::parsing::types::type_from_parsed(&t, i)?)))})))
             .add_var(
                 "pop".to_string(),
                 Data::new(data::function::Function {
@@ -202,7 +206,10 @@ impl MersType for ListT {
             .is_some_and(|v| self.0.is_same_type_as(&v.0))
     }
     fn is_included_in_single(&self, target: &dyn MersType) -> bool {
-        self.is_same_type_as(target)
+        target
+            .as_any()
+            .downcast_ref::<Self>()
+            .is_some_and(|v| self.0.is_included_in(&v.0))
     }
     fn subtypes(&self, acc: &mut Type) {
         for t in self.0.subtypes_type().types {
@@ -234,7 +241,7 @@ impl Display for List {
 }
 impl Display for ListT {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "[{}]", self.0)?;
+        write!(f, "List<{}>", to_string_literal(&self.0.to_string(), '>'))?;
         Ok(())
     }
 }

mers_lib/src/program/configs/with_multithreading.rs

@@ -7,6 +7,7 @@ use std::{
 use crate::{
     data::{self, Data, MersData, MersType, Type},
     errors::CheckError,
+    parsing::{statements::to_string_literal, Source},
     program::{self, run::CheckInfo},
 };
 
@@ -19,7 +20,10 @@ impl Config {
     pub fn with_multithreading(self) -> Self {
         self.add_type(
             "Thread".to_string(),
-            Type::new(ThreadT(Type::empty_tuple())),
+            Err(Arc::new(|s, i| {
+                let t = crate::parsing::types::parse_type(&mut Source::new_from_string_raw(s.to_owned()))?;
+                Ok(Arc::new(ThreadT(crate::parsing::types::type_from_parsed(&t, i)?)))
+            })),
         )
         .add_var(
             "thread".to_string(),
@@ -170,6 +174,6 @@ impl Display for Thread {
 }
 impl Display for ThreadT {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "<Thread>")
+        write!(f, "Thread<{}>", to_string_literal(&self.0.to_string(), '>'))
     }
 }

mers_lib/src/program/parsed/as_type.rs

@@ -0,0 +1,38 @@
+use crate::{
+    errors::{CheckError, SourceRange},
+    parsing::types::ParsedType,
+    program::{self},
+};
+
+use super::{CompInfo, MersStatement};
+
+#[derive(Debug)]
+pub struct AsType {
+    pub pos_in_src: SourceRange,
+    pub statement: Box<dyn MersStatement>,
+    pub as_type: Vec<ParsedType>,
+    pub type_pos_in_src: SourceRange,
+    pub expand_type: bool,
+}
+
+impl MersStatement for AsType {
+    fn has_scope(&self) -> bool {
+        false
+    }
+    fn compile_custom(
+        &self,
+        info: &mut crate::info::Info<super::Local>,
+        comp: CompInfo,
+    ) -> Result<Box<dyn program::run::MersStatement>, CheckError> {
+        Ok(Box::new(program::run::as_type::AsType {
+            pos_in_src: self.pos_in_src,
+            statement: self.statement.compile(info, comp)?,
+            as_type: self.as_type.clone(),
+            type_pos_in_src: self.type_pos_in_src,
+            expand_type: self.expand_type,
+        }))
+    }
+    fn source_range(&self) -> SourceRange {
+        self.pos_in_src
+    }
+}

mers_lib/src/program/parsed/custom_type.rs

@@ -0,0 +1,46 @@
+use std::{fmt::Debug, sync::Arc};
+
+use crate::{
+    errors::{CheckError, SourceRange},
+    parsing::types::{type_from_parsed, ParsedType},
+};
+
+use super::{CompInfo, Info, MersStatement};
+
+pub struct CustomType {
+    pub pos_in_src: SourceRange,
+    pub name: String,
+    pub source: Result<Vec<ParsedType>, Box<dyn MersStatement>>,
+}
+impl MersStatement for CustomType {
+    fn compile_custom(
+        &self,
+        info: &mut Info,
+        comp: CompInfo,
+    ) -> Result<Box<dyn crate::program::run::MersStatement>, CheckError> {
+        let src = match &self.source {
+            Ok(p) => Ok(p.clone()),
+            Err(s) => Err(s.compile(info, comp)?),
+        };
+        Ok(Box::new(crate::program::run::custom_type::CustomType {
+            pos_in_src: self.pos_in_src,
+            name: self.name.clone(),
+            source: Box::new(move |ci| match &src {
+                Ok(parsed) => Ok(Ok(Arc::new(type_from_parsed(parsed, ci)?))),
+                Err(statement) => Ok(Ok(Arc::new(statement.check(&mut ci.clone(), None)?))),
+            }),
+        }))
+    }
+    fn has_scope(&self) -> bool {
+        false
+    }
+    fn source_range(&self) -> SourceRange {
+        self.pos_in_src
+    }
+}
+
+impl Debug for CustomType {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "type {} <...>", self.name)
+    }
+}

mers_lib/src/program/parsed/mod.rs

@@ -5,6 +5,8 @@ use crate::{
     info,
 };
 
+#[cfg(feature = "parse")]
+pub mod as_type;
 #[cfg(feature = "parse")]
 pub mod assign_to;
 #[cfg(feature = "parse")]
@@ -12,6 +14,8 @@ pub mod block;
 #[cfg(feature = "parse")]
 pub mod chain;
 #[cfg(feature = "parse")]
+pub mod custom_type;
+#[cfg(feature = "parse")]
 pub mod function;
 #[cfg(feature = "parse")]
 pub mod r#if;
@@ -70,6 +74,7 @@ pub struct Local {
 impl info::Local for Local {
     type VariableIdentifier = String;
     type VariableData = (usize, usize);
+    type Global = ();
     fn init_var(&mut self, id: Self::VariableIdentifier, value: Self::VariableData) {
         self.vars_count += 1;
         self.vars.insert(id, value);

mers_lib/src/program/parsed/variable.rs

@@ -22,20 +22,29 @@ impl MersStatement for Variable {
         info: &mut crate::info::Info<super::Local>,
         comp: CompInfo,
     ) -> Result<Box<dyn program::run::MersStatement>, CheckError> {
+        let init_and_ignore = comp.is_init && self.var == "_";
         if comp.is_init {
-            info.init_var(
+            if !init_and_ignore {
-                self.var.clone(),
+                info.init_var(
-                (
+                    self.var.clone(),
-                    info.scopes.len() - 1,
+                    (
-                    info.scopes.last().unwrap().vars_count,
+                        info.scopes.len() - 1,
-                ),
+                        info.scopes.last().unwrap().vars_count,
-            )
+                    ),
+                );
+            }
         }
         Ok(Box::new(program::run::variable::Variable {
             pos_in_src: self.pos_in_src,
             is_init: comp.is_init,
-            is_ref: comp.is_init || self.is_ref,
+            is_ref_not_ignore: if comp.is_init {
-            var: if let Some(v) = info.get_var(&self.var) {
+                !init_and_ignore
+            } else {
+                self.is_ref
+            },
+            var: if init_and_ignore {
+                (usize::MAX, usize::MAX)
+            } else if let Some(v) = info.get_var(&self.var) {
                 *v
             } else {
                 return Err(CheckError::new()

mers_lib/src/program/run/as_type.rs

@@ -0,0 +1,78 @@
+use colored::Colorize;
+
+use crate::{
+    data::{Data, MersType, Type},
+    errors::{error_colors, CheckError, SourceRange},
+    parsing::types::ParsedType,
+};
+
+use super::MersStatement;
+
+#[derive(Debug)]
+pub struct AsType {
+    pub pos_in_src: SourceRange,
+    pub statement: Box<dyn MersStatement>,
+    pub as_type: Vec<ParsedType>,
+    pub type_pos_in_src: SourceRange,
+    /// if false, only return an error if type doesn't fit, but don't expand type if it fits
+    pub expand_type: bool,
+}
+
+impl MersStatement for AsType {
+    fn check_custom(
+        &self,
+        info: &mut super::CheckInfo,
+        init_to: Option<&Type>,
+    ) -> Result<Type, CheckError> {
+        if init_to.is_some() {
+            return Err("can't init to statement type AsType (move type annotations from initialization to statement?)".to_string().into());
+        }
+        let return_type = self.statement.check(info, None)?;
+        let as_type =
+            crate::parsing::types::type_from_parsed(&self.as_type, info).map_err(|e| {
+                CheckError::new()
+                    .src(vec![(
+                        self.type_pos_in_src,
+                        Some(error_colors::BadTypeFromParsed),
+                    )])
+                    .err(e)
+            })?;
+        if !return_type.is_included_in(&as_type) {
+            return Err(CheckError::new()
+                .src(vec![
+                    (self.pos_in_src, None),
+                    (
+                        self.type_pos_in_src,
+                        Some(error_colors::AsTypeTypeAnnotation),
+                    ),
+                    (
+                        self.statement.source_range(),
+                        Some(error_colors::AsTypeStatementWithTooBroadType),
+                    ),
+                ])
+                .msg(format!(
+                    "Type must be included in {}, but the actual type {} isn't.",
+                    as_type
+                        .to_string()
+                        .color(error_colors::AsTypeTypeAnnotation),
+                    return_type
+                        .to_string()
+                        .color(error_colors::AsTypeStatementWithTooBroadType)
+                )));
+        }
+        Ok(if self.expand_type {
+            as_type.clone()
+        } else {
+            return_type
+        })
+    }
+    fn run_custom(&self, info: &mut super::Info) -> Data {
+        self.statement.run(info)
+    }
+    fn has_scope(&self) -> bool {
+        false
+    }
+    fn source_range(&self) -> SourceRange {
+        self.pos_in_src
+    }
+}

mers_lib/src/program/run/custom_type.rs

@@ -0,0 +1,63 @@
+use std::{fmt::Debug, sync::Arc};
+
+use crate::{
+    data::{Data, MersType, Type},
+    errors::{CheckError, SourceRange},
+};
+
+use super::{CheckInfo, Info, MersStatement};
+
+pub struct CustomType {
+    pub pos_in_src: SourceRange,
+    pub name: String,
+    pub source: Box<
+        dyn Fn(
+                &CheckInfo,
+            ) -> Result<
+                Result<
+                    Arc<dyn MersType>,
+                    Arc<
+                        dyn Fn(&str, &CheckInfo) -> Result<Arc<dyn MersType>, CheckError>
+                            + Send
+                            + Sync,
+                    >,
+                >,
+                CheckError,
+            > + Send
+            + Sync,
+    >,
+}
+
+impl MersStatement for CustomType {
+    fn check_custom(
+        &self,
+        info: &mut CheckInfo,
+        init_to: Option<&Type>,
+    ) -> Result<Type, CheckError> {
+        if init_to.is_some() {
+            return Err("can't init to `type` statement".to_string().into());
+        }
+        let t = (self.source)(info)?;
+        info.scopes
+            .last_mut()
+            .unwrap()
+            .types
+            .insert(self.name.clone(), t);
+        Ok(Type::empty_tuple())
+    }
+    fn run_custom(&self, _info: &mut Info) -> Data {
+        Data::empty_tuple()
+    }
+    fn has_scope(&self) -> bool {
+        false
+    }
+    fn source_range(&self) -> SourceRange {
+        self.pos_in_src
+    }
+}
+
+impl Debug for CustomType {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "<CustomType>")
+    }
+}

mers_lib/src/program/run/mod.rs

@@ -1,14 +1,17 @@
 use std::{
+    collections::HashMap,
     fmt::Debug,
     sync::{Arc, RwLock},
 };
 
 use crate::{
-    data::{self, Data, Type},
+    data::{self, Data, MersType, Type},
     errors::{CheckError, SourceRange},
     info,
 };
 
+#[cfg(feature = "run")]
+pub mod as_type;
 #[cfg(feature = "run")]
 pub mod assign_to;
 #[cfg(feature = "run")]
@@ -16,6 +19,8 @@ pub mod block;
 #[cfg(feature = "run")]
 pub mod chain;
 #[cfg(feature = "run")]
+pub mod custom_type;
+#[cfg(feature = "run")]
 pub mod function;
 #[cfg(feature = "run")]
 pub mod r#if;
@@ -65,13 +70,26 @@ pub type CheckInfo = info::Info<CheckLocal>;
 pub struct Local {
     vars: Vec<Arc<RwLock<Data>>>,
 }
-#[derive(Default, Clone, Debug)]
+#[derive(Default, Clone)]
 pub struct CheckLocal {
     vars: Vec<Type>,
+    pub types: HashMap<
+        String,
+        Result<
+            Arc<dyn MersType>,
+            Arc<dyn Fn(&str, &CheckInfo) -> Result<Arc<dyn MersType>, CheckError> + Send + Sync>,
+        >,
+    >,
+}
+impl Debug for CheckLocal {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "CheckLocal {:?}, {:?}", self.vars, self.types.keys())
+    }
 }
 impl info::Local for Local {
     type VariableIdentifier = usize;
     type VariableData = Arc<RwLock<Data>>;
+    type Global = ();
     fn init_var(&mut self, id: Self::VariableIdentifier, value: Self::VariableData) {
         let nothing = Arc::new(RwLock::new(Data::new(data::bool::Bool(false))));
         while self.vars.len() <= id {
@@ -104,6 +122,7 @@ impl info::Local for Local {
 impl info::Local for CheckLocal {
     type VariableIdentifier = usize;
     type VariableData = Type;
+    type Global = ();
     fn init_var(&mut self, id: Self::VariableIdentifier, value: Self::VariableData) {
         while self.vars.len() <= id {
             self.vars.push(Type::empty());

mers_lib/src/program/run/variable.rs

@@ -11,7 +11,8 @@ use super::MersStatement;
 pub struct Variable {
     pub pos_in_src: SourceRange,
     pub is_init: bool,
-    pub is_ref: bool,
+    // if `is_init` is true, this must also be true unless using the "ignore" `_` pattern
+    pub is_ref_not_ignore: bool,
     pub var: (usize, usize),
 }
 
@@ -25,14 +26,22 @@ impl MersStatement for Variable {
         init_to: Option<&Type>,
     ) -> Result<data::Type, super::CheckError> {
         if self.is_init {
-            while info.scopes[self.var.0].vars.len() <= self.var.1 {
+            if self.is_ref_not_ignore {
-                info.scopes[self.var.0].vars.push(Type::empty());
+                while info.scopes[self.var.0].vars.len() <= self.var.1 {
+                    info.scopes[self.var.0].vars.push(Type::empty());
+                }
+                info.scopes[self.var.0].vars[self.var.1] = init_to
+                    .expect("variable's is_init was true, but check_custom's assign was None? How?")
+                    .clone();
+            } else {
+                return Ok(Type::new(data::reference::ReferenceT(
+                    init_to
+                        .expect("var's is_init was true, but init_to was None???")
+                        .clone(),
+                )));
             }
-            info.scopes[self.var.0].vars[self.var.1] = init_to
-                .expect("variable's is_init was true, but check_custom's assign was None? How?")
-                .clone();
         }
-        let val = if self.is_ref {
+        let val = if self.is_ref_not_ignore {
             Type::new(data::reference::ReferenceT(
                 info.scopes[self.var.0].vars[self.var.1].clone(),
             ))
@@ -43,13 +52,20 @@ impl MersStatement for Variable {
     }
     fn run_custom(&self, info: &mut super::Info) -> Data {
         if self.is_init {
-            let nothing = Arc::new(RwLock::new(Data::new(data::bool::Bool(false))));
+            if self.is_ref_not_ignore {
-            while info.scopes[self.var.0].vars.len() <= self.var.1 {
+                let nothing = Arc::new(RwLock::new(Data::new(data::bool::Bool(false))));
-                info.scopes[self.var.0].vars.push(Arc::clone(&nothing));
+                while info.scopes[self.var.0].vars.len() <= self.var.1 {
+                    info.scopes[self.var.0].vars.push(Arc::clone(&nothing));
+                }
+                info.scopes[self.var.0].vars[self.var.1] = nothing;
+            } else {
+                // (reference to) data which will never be referenced again
+                return Data::new(data::reference::Reference(Arc::new(RwLock::new(
+                    Data::empty_tuple(),
+                ))));
             }
-            info.scopes[self.var.0].vars[self.var.1] = nothing;
         }
-        if self.is_ref {
+        if self.is_ref_not_ignore {
             Data::new(data::reference::Reference(Arc::clone(
                 &info.scopes[self.var.0].vars[self.var.1],
             )))