Plasma Type System Design

Paul Bone
<paul@plasmalang.org>
version 0.1, April 2017
Draft. Copyright © 2017 Plasma Team License: CC BY-SA 4.0
Table of Contents

This is a design/set of ideas that I’m considering for Plasma’s type system. it is very much a draft. It is more or less an aide to help me write down my ideas, work them through and eventually refine then and make them part of the reference manual.

Starting with a type system such as the basic parts of Haskell’s or Mercury’s specifically:

  • Discriminated unions / ADTs

  • Polymorphism

  • Abstract types

  • Existential types (later)

  • More kinds (later)

Starting with this I have been considering the kinds of subtyping that OCaml can do, it’s pretty neat. But recently I read a blog post by Troels Henriksen about structural typing and record syntax for Futhark. It made me consider this more deeply, and now I have the following design in mind.

Basic stuff

We can define our own types, such as enums:

type Suit = Hearts | Diamonds | Spades | Clubs

Or structures, this type has a single struct with named fields.

type PlayingCard = Card ( suit : Suit, number : Number )

A combination of the above, a PlayingCard is either an ordinary card or a joker. An ordinary card has fields.

type PlayingCard = OrdinaryCard ( suit : Suit, number : Number )
                 | Joker

Types are polymorphic, they may take type parameters. Identifiers beginning with upper case letters denote type names and constructor names. Identifiers beginning with lower-case letters denote type variables.

type Tree(k, v) = EmptyTree
                | Node (
                    key     : k,
                    value   : v,
                    left    : Tree(k, v),
                    right   : Tree(k, v)
                )

A type alias, ID is now another word for Int. (XXX this needs revisiting).

type_alias ID = Int

It’s often more useful to alias something more complex.

type_alias Name = String
type_alias NameMap = Map(ID, Name)

Type aliases can take parameters:

type_alias IDMap(x) = Map(ID, x)

Terminology

Before we can go further, I want to pin down some terminology.

A type has a name and some parameters. Eg Int or Map(k, v)

A type declaration has multiple parts:

type Tree(k, v) = EmptyTree
                | Node (
                    key     : k,
                    value   : v,
                    left    : Tree(k, v),
                    right   : Tree(k, v)
                )

A type declaration is made of a left hand side and a right hand side (either side of the =). The left-hand-side contains a type name Tree it’s parameters k and v which creates the type Tree(k, v). This is also a type expression but we’ll get to that later.

TODO: kinds.

The right hand side is a structure expression. A structure expression is normally made up of structures structures separated by | symbols (meaning "or"). But could be made of other things such as a reference to another type (type(TypeExpr)). Each structure is made up of an optional structure (the bit in the parens) and a tag (its name). The structure is optional, if there are no fields then one should not write empty parens. An untagged structure, which we will see later, is written with the _ for its tag. When an untagged structure is used, there must be exactly one structure in the type. Other languages often call the tagged structures data constructors, and the untagged ones tuples, but I hope that calling them both structures, and allowing untagged structures to use field names and share some syntax will be good.

Note
The word tag is overloaded, it also refers to an implementation detail for discriminated unions, that’s not what we’re referring to here.

Each field in the structure has a name and a type. The type is any type expression. The field name is optional.

Finally type expressions refer either a type like Map(k, v) (including abstract types), or multiple type expressions separated by |, or arbitrary structure expressions when wrapped in struct(). The struct() wrapper avoids ambiguity between tags and types. Likewise, and not mentioned above, structure expressions can refer to whole types with type().

Any type variables appearing in structures (on the RHS of the = in the type declaration, must appear exactly once on the LHS. This may need to change for existential types (TODO).

Subtyping / constructors are "shared"

type TypeA = A | B
type TypeB = A | B | C

A function that accepts parameters of type TypeA, cannot be passed values of TypeB. But a function accepting parameters of type TypeB can be passed values of TypeA.

This works along with type inference. This function:

func my_func() -> _
{
    return A
}

Is known to return A which is covered by either TypeA or TypeB. This functions inferred return type will be struct(A). Which we know we can pass as either TypeA or TypeB. Care will need to be taken when generating error messages.

Likewise, if my_func was defined as:

func my_func(...) -> _
{
    if (...) {
        return A
    } else {
        return B
    }
}

Then it would be inferred as returning TypeA since we already have a name for struct(A | B).

Types defined in separate modules outside the view of each-other can’t share tags. This is not the limitation it seems, since usually when such a feature is required it is to extend, or in some cases reduce, the constructor symbols of an existing known type.

type AdvancedNode(a) = type(BasicNode(a))
                     | AdvancedStruct (
                        ...
                     )
Note
See below for how recursive types are handled.

Magic type tagging.

I think I saw this in Perl 6.

func do_something(...) -> Result | Error
{
    ...
}

That’s easy provided that both these types have all their structures tagged, but Ints, Strings, etc don’t work like that (each Int, String etc is like an alternative tag in an infinite or really large set of tags).

Where all the types in a type expression are named, then they may also be switched by type, rather than just value. (the compiler tags and probably boxes them internally).

func print(x : Int | String) -> String
{
    return switch_type(x) {
        Int -> int_to_string(x)
        String -> "\"" ++ x ++ "\""
    }
}

Ordering

This kind of subtyping must work via an ordering. There is a partial ordering over all types, types that refer to something more specific are greater in this ordering. Therefore: A | B > A | B | C.

Being a partial ordering some types cannot be compared, eg: A | B and B | C. This means that neither is a subtype of the other.

Adding fields.

This can vary depending upon how programmers express deconstructions. Usually a deconstruction is (semantically) a match statement.

match (a) {
    A (x) -> { return x }
}

The equivlient deconstruction might look like:

A(x) = a
return x

This matches A with a single field. It would also match any A with at least one field, extracting only the first. Let’s make a new structure expression for that: A/1, this kind of type expression wont appear in programs or even error messages, but we need it here to discuss subtyping and ordering.

But fields can also be extracted or structures can be matched using field names.

# Field selection.
return a.field1

# Match with fields.
match (a) {
    A(x = field1) -> { return x }
}

Therefore we also need to talk about subtyping with regard to fields and their types. We write the type of a in these as: struct(A(field1 : t)) (t is currently abstract). It’s more correct to say that the first is struct(_(field1 : t)) since the constructor symbol isn’t mentioned.

Any use of a constructor, such as the match statement but not the field selection, requires a type to have already been declared. This will make more sense later with tagless structures. For now lets just say we require a type to exist.

For example, either TypeA or TypeB match the above usages.

type TypeA = A (
                field1 : Int
             )
type TypeB = A (
                field1 : Int,
                field2 : String
             )

Ordering with fields

A structure expression with more fields is greater than one with fewer: struct(A/2) > struct(A/1).

A structure expression with a superset anther’s fields is greater: struct(A( field1 : Int, field2 : String )) > struct(A ( field1 : Int )), Or: TypeB > TypeA

A structure expression with a constructor and with the same or a superset of anothers fields is greater: struct(A( field1 : Int )) > struct(_(field1 : Int)),

Fields are compared by name and type. A field whose type is greater than the corresponding field of another type, is greater. This makes ordering composable, and work as desired on recursive types.

Structures whose fields are neither a set or superset cannot be ordered. Structure expressions with numbers cannot be ordered with those by types.

Widening is performed when a type or a constructor symbol is named, or if no ordering is found between two types, they are widened in an attempt to find a common type. Widening allows more programs to be well typed, makes the type system easier to use, however it makes type more specific than strictly necessary.

A type expression such as the above is widened to the disjunction of the equal-least specific types matching the expression. Nevertheless we discussed ordering of these expressions so that we can determine ordering of types and which type an expression may be widened to. Widening must also respect the types created by type expressions the developer writes, such as in the declaration of a function:

func do_something(...) -> Result | Error
{
    ...
}

In this case Result | Error is considered for widening.

Untagged structures

An untagged structure can be used, but without any other structures within the same type. The missing tag must be written with _.

type TypeC = _(
                field1: Int,
                field3: Bool
             )

Uses of untagged structures are not widened unless combined with a named structure or type containing a named structure. This makes them feel more "dynamic" although they simply use type inference heavily. For example:

dict_from_kv_list(map(\x -> _( key = get_id(x), value = x ), list))

TODO: I think : is the best operator here, but it conflicts badly with "has type". = is also okay. Arrows can be a problem as the directions matter, and sometimes you want them one way or the other.

Without needing to declare a type, using an untagged structure like this implicitly creates one for us.

Syntax

We’ve already seen some syntax above. But I’d like to expand on that now.

Type structures use parentheses ( ), fields are separated by commas and the tag may either be an identifier starting with a capital letter or the _ symbol. Each field separates it’s field name from the field type (in declarations) with a : (meaning "type of").

Deconstructions may be done with fields (using a =, with the new variable on the left) or by position. Constructions are done either positionally or with the field name on the left.

Selection is performed using the . operator, the general scoping operator.

x = point.x

This may be chained for nested structures.

a = struct.field1.next_field.other_field

Field update (in an expression) is introduced via the join symbol: |

new_struct = _( old_struct | field = new_value )

Nested fields may also be updated, updating all the structures along the way.

new_struct = _( old_struct | field.next_field.following_field = new_value )

Multiple fields may be updated in one expression. These expressions will introduce fields if necessary (subject to type checking). These will also work for tagged structs.

A syntas sugar for state variables in statements will probably be introduced.

Syntax TODOs

Projection / Explosion / Filtering

Additional syntax could be created to merge structs (exploding one into a series of updates to another). Filtering (technically projection) may also have some use, either to satisfy the type system or to free memory.

Lenses

Lenses (optics?) provide some additional power not provided here. It’d be nice to handle that if possible.

Co-variance and contra-variance

TODO: I will need to address this to support arrays.