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https://github.com/frankbro/ordo

Ordo: A minimalist language with row polymorphism
https://github.com/frankbro/ordo

compiler programming-language records row-polymorphism variants

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Ordo: A minimalist language with row polymorphism

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![example workflow](https://github.com/frankbro/ordo/actions/workflows/rust.yml/badge.svg)



# Ordo



Latin : ordo



English : a methodical series, arrangement, or order; regular line, row, or series



## Introduction



A minimal statically-typed programming language focused around row polymorphism, which allows to infer structural types: records and variants.



* Polymorphic types:

```ocaml

ordo>>> let f(x) = x

forall a => a -> a

fun(x)

```



* Records:

```ocaml

ordo>>> let record = { x = 1, y = 2 }

{x: int, y: int}

{x: 1, y: 2}

```



* Variants:

```ocaml

ordo>>> let variant = :variant 1

forall ra. (ra\variant) => [variant: int | ra]

:variant 1

```



The extra syntax at the end of the variant means it is an open variant. Rows (record or variants) can be open or closed. By default, record literals are closed but variant literals are open. Rows also infer their restrictions. 



Here, `forall` signifies that generic types will be given next. We might also have a portion between parenthesis, indicating row restrictions. In this case, for a generic type `ra`, which we know is a row because it's followed by the restriction that this row should not have the value `variant`. Next we finally have the actual type of our expression, we know it's a variant because it's surrounded by `< >`, whereas records are surrounded by `{ }`. The type of the expression is a variant that extends the generic row `r` by having a new case that is specified: `variant` of type `int`.



## Record



* Record initialization

```ocaml

ordo>>> {}

{}

{}

```

* Record extension adds fields to the record

```ocaml

ordo>>> { x = 1 | {} }

{x: int}

{x: 1}

```

* Record initialization, being sugar for record extension

```ocaml

ordo>>> { x = 1 } == { x = 1 | {} }

bool

true

```

* The order of insertion of labels is not important

```ocaml

ordo>>> { x = 1, y = 2 } == { y = 2, x = 1 }

bool

true

```

* Record restriction removes fields from a record

```ocaml

ordo>>> { x = 1 }\x == {}

bool

true

```

* Record cannot have the same label twice

```ocaml

ordo>>> { x = 1, x = 2 }

error: parser: duplicate label: x

```



In fact, we can see this property being infered by the type system and shown:



```ocaml

ordo>>> let f(r) = { y = 0 | r }

forall ra. (ra\y) => {ra} -> {y: int | ra}

fun(r)

ordo>>> f({ x = 0 })

{x: int, y: int}

{x: 0, y: 0}

```



The signature here specifies that the function takes a row that does not contain the field `y`. If you provide a record with the field `y` already it in, the type inference won't let you.



```ocaml

ordo>>> f({ y = 1 })

error: infer: row constraint failed for label: y

```



* Structual pattern matching and even make sure it is valid via type checking:



```ocaml

ordo>>> let { x = x } = { x = 1 }

{x: int}

{x: 1}

ordo>>> x

int

1

ordo>>> let { y = y } = { x = 1 }

error: infer: missing label: y

```



* Record matching is open



While record literals are closed rows, record matching is open:



```ocaml

ordo>>> let record = { x = 0 }

{x: int}

{x: 0}

ordo>>> let f({ x = x }) = x

forall a ra. (ra\x) => {x: a | ra} -> a

fun({x: x})

```



* Sugar for matching and creation



Records can either directly assign fields to variables in a shorthand syntax or capture variables via closure and set them automatically to their variable name as label:



```ocaml

ordo>>> let f({x,y}) = x + y

forall ra. (ra\x\y) => {x: int, y: int | ra} -> int

fun({x: x, y: y})

ordo>>> let x = 1

int

1

ordo>>> let y = 2

int

2

ordo>>> f({x,y})

int

3

```



## Variant



The few new expressions introduced for variants:



* Variant creation



```ocaml

ordo>>> let variant = :variant 1

forall ra. (ra\variant) => [variant: int | ra]

:variant 1

```



* Variant elimination



Variant can be eliminated via pattern matching. While variant literals are open rows, we can deduce if the input if open on closed based on the fact that a default case is provided or not.



```ocaml

ordo>>> let default_with(default, value) = match value { :some value -> value, :none x -> default }

forall a b => (a, [none: b, some: a]) -> a

fun(default, value)

```



In this case, we did not provide a default case and therefore, the variant was infered to be closed. However, if we wrote it this way:



```ocaml

ordo>>> let is_success(v) = match v { :success a -> true , otherwise -> false }

forall a ra. (ra\success) => [success: a | ra] -> bool

fun(v)

```



This is useful to make sure which variant can be passed in.



```ocaml

ordo>>> is_success(:fail 0)

bool

false

ordo>>> default_with(1, :fail 0)

error: infer: missing label: fail

```