Lawvere

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Lawvere — Tutorial

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Lawvere

A categorical programming language with effects

Install • Tutorial • Editor support • Development

Very work-in-progress

(.playerA .points - .playerB .points)
{ leader = (> 0) [ true = "A", false = "B"],
  delta  = abs show }
"Player {.leader} is winning by {.delta} points!"

The Lawvere language (and the executable bill) is named after William Lawvere.

Tutorial

REPL

Once installed, start a Lawvere REPL (Read-Eval-Print-Loop) with bill -i:

$ bill -i
--------------
Lawvere v0.0.0
--------------
> 40 + 2
42
> "hello"
"hello"

(Or cabal run bill -- -i or stack exec bill -- -i if bill isn't install.)

bill can also be given a file: bill -i example.law. By omitting the -i flag, the main arrow is executed directly and the REPL is not started. In the REPL, :r will reload the loaded file, and :q will terminate the session. This README file is a literate Lawvere script, so you can load it and try out the examples:

$ bill -i README.md
Check OK!
> answer + 1
43

Basic types

Values of basic types are written as in other programming languages, e.g. 42 and "hello world". But in Lawvere, everything is an arrow (Lawvere's equivalent of a function), so that these actually denote constant arrows. For example, 42 denotes the arrow which is constantly 42:

ar answer : {} --> Int = 42

The above code defines an arrow using the ar keyword. The arrow has source {} (which is the syntax for the unit type) and target Int.

When the REPL accepts an input, it actually executes it (with the Haskell evaluator) on the unit input. So for example inputting incr (which expects an Int) will result in an error.

Lawvere also has support for basic arithmetic and comparisons. These are operations on arrows, for example f + g forms the pointwise addition of arrows f and g.

Composition

The main way to build up larger programs from smaller ones is by using composition. The syntax for this is very lightweight - it's simply whitespace! That is, f g denotes the composition of f and g. If you are coming from Haskell, note that this does not correspond to ., but to >>>, that is, f comes first, then g.

(If you know Forth composition and literals as constant functions will feel familiar.)

To illustrate this we can use the built-in arrow incr, which increments an integer:

ar plus3 : Int --> Int = incr incr incr

ar fourtyFive : {} --> Int = 42 plus3

will output 45.

To run this, create a file with the above contents and use bill:

$ bill -i test.law
--------------
Lawvere v0.0.0
--------------
checking..
Check OK!
> fourtyFive
45
> fourtyFive plus3 incr
49

Note: The checker is a work-in-progress and is far from complete.

The identity arrow is called identity, but you can also write it with nothing at all (or whitespace). So the mathematical function [x ↦ x * x + 1] can be written identity * identity + 1, or simply * + 1:

ar squarePlusOne : Int --> Int = identity * identity + 1

ar squarePlusOne' : Int --> Int = * + 1
$ bill -i test.law
> 2 squarePlusOne'
5

Products

Datatypes in lawvere are called objects (since they correspond to objects in category theory -- as arrows, as you might have guessed, correspond to morphisms). We define a new object Point with the keyword ob. If the object is a product type (or 'struct', or 'record'), specify it using braces:

ob Base Point = { x: Float, y: Float }

The arrow which projects out the x component from Point is written .x. (Think of the foo.x notation that is usual in other programming languages, except without anything preceding the dot.)

$ bill -i
> { user = { name = "Mina", age = 2 }, req = { format = "json" } } .user .name
"Mina"

To create arrow to a product, we specify, again using braces, arrows to each component of the product (in categorical terms, a cone) . For example,

ar somePoint : {} --> Point =
  { x = 2.3, y = 4.6 }

This works because 2.3 and 4.6 are arrows of type {} --> Float, and the braces syntax uses arrows which all have the same source.

In general, arrows of type X --> { a: A, b: B, c: C, ... } can be written as

{ a = f, b = g, ... }

if f : X --> A, g : X --> B, h : X --> C, etc.

Here's a fuller example of using products:

ob Base Point = { x: Float, y: Float }

ar linFun : Point --> Float = 2.0 * .x + 3.0 * .y

ar someNum : {} --> Float =
  { x = 2.3, y = 4.6 } linFun

someNum is then 18.4.

The empty product object (in categorical terms, the terminal object) is written as {}, and the unique arrow to it is also {}. If there is any ambiguity (which occurs when defining functors) then you can use {:} and {=}.

By using parentheses instead of braces, the components are positional rather than named. In this case the projections are .1, .2, etc. Using a positional product for Point the previous program would be:

ob Base PointPos = (Float, Float)

ar linFunPos : PointPos --> Float = 2.0 * .1 + 3.0 * .2

ar someNumPos : {} --> Float =
  (2.3, 4.6) linFunPos

String interpolation

A string can contain interpolated expressions. For example, "Name: {f}, Age: {g}" denotes an arrow A --> String as long as both f and g are also morphisms A --> String.

The program:

ar james : {} --> String =
  { name= "James", hobby= "playing Go" } "{.name} likes {.hobby}."

will result in "James likes playing Go."

Sums

We can define sum types too. For instance, booleans:

ob Base Bool = [ true: {}, false: {} ]

Using square brackets we define a sum type with two summands, true and false, each with {} as payload.

Sum types come equipped with constructors (injection). The constructor into the component with name foo is denoted foo., simply mirroring the notation for projections.

In order to define some simple boolean functions, we'll need to learn how to map from sums. This is like pattern matching, specifying an arrow for each summand (and thus, in categorical language, a cocone). This is similar to cones, except using square brackets instead of braces. To illustrate this let's define the negation function:

ar not : Bool --> Bool
  = [ true  = false.,
      false = true. ]

In words, we split the arrow into two cases. In the first case (on the true component) we use false. constructor, on the other component we use true..

In general, to specify an arrow:

[ a: A, b: B, c: C, ... ] --> X

one uses a cocone

[ a = f, b = g, c = h, ... ]

where f : A --> X, g : B --> X, h : C --> X, etc.

Distribution

Continuing with boolean functions, let's try to define the and function:

ar and : {x: Bool, y: Bool} --> Bool = ?

This is an arrow to a sum (Bool), so we can't use a cocone, and from a product ({x : Bool, y: Bool }), so we can't use a cone---are we stuck? Intuitively we want to inspect one of the two arguments (x or y) in order to continue. For this we will use the distributor @x. To understand what this does, first let's re-write {x : Bool, y : Bool} by expanding the definition of Bool at the x summand:

{ x: [ true: {}, false: {}], y: Bool }

The type of @x is:

@x : { x: [ true: {}, false: {}], y: Bool } --> [ true: { x: {}, y: Bool}, false: { x: {}, y: Bool } ]

The morphism @x transforms the product into a sum; a sum with the same summand names as the sum in the component it targets. So in this case we end up with a sum with summands true and false, and the x component contains the unwrapped payload for the original sum at x (in this case they are both {}).

Using this we can define and as follows:

ar and : {x : Bool, y : Bool} --> Bool =
  @x [ true  = .y,
       false = {} false. ]

In words: "Perform a case analysis on x, if x is true, then return y, otherwise return false". Note the similarity with the equivalent Elm program (Haskell doesn't have anonymous records, making the comparison less clear), even though Lawvere has no variables or λs:

and : { x : Bool, y : Bool } -> Bool
and input =
  case input.x of
    True  -> input.y
    False -> False

Summing over a list

In this example we'll sum up a list of values.

First we'll define lists of Ints (we'll learn how to define the list functor later, which means we don't need to define a new object for each possible object of elements):

ob Base ListI =
  [ empty: {},
    cons:  { head: Int, tail: ListI }
  ]

An example list can be built up by composing morphisms together:

ar aFewPrimes : {} --> ListI =
  empty.
  { head = 2, tail = } cons.
  { head = 3, tail = } cons.
  { head = 5, tail = } cons.

Note that in { head = 2, tail = }, the arrow being used at the tail component is the identity. This could also be written { head = 2, tail = identity}. Another thing to note is that the 2 being used here doesn't have source {}, indeed all integer literals actually have type forall a. a --> Int (and similarly for other scalars). This saves one from having to write {} 2. Like Haskell, Lawvere has some syntactic sugar for list-building:

ar morePrimes : {} --> ListI =
  #(2, 3, 5, 7, 11)

We can sum over a list using a cocone:

ar sum : ListI --> Int =
  [ empty = 0,
    cons  = .head + .tail sum ]

In words: If the list is empty, then return 0. Otherwise take the head, and the sum of the .tail, and + them together.

> morePrimes sum
28

Effects

(Very WIP)

Lawvere is is a pure language but allows programming with effects using free Freyd categories, much like Haskell is pure but allows programming with effects using monads or arrows. In fact Freyd categories and arrows are very similar, e.g. see Categorical semantics for arrows.

I/O

The IO effect is built-in. Here is an example of a morphism which performs I/O:

ar Base[IO] hello : {} --> String =
  ~"What is your name?" putLine
  getLine
  ~"Hello {}" putLine

To run this, one must use the io functor:

ar InputOutput helloIO : {} --> {} =
  io(hello)

This will print What is your name, wait for the user to input their name, and then greet them.

Cones ({..}) are not permitted in effectful morphisms, but one can still perform effects at a single component. Here is a program which asks for 2 pieces of user input:

// Turn a question into an answer.
ar Base[IO] ask : String --> String =
  putLine getLine

// Ask some questions and then print a greeting.
ar Base[IO] greet : {} --> {} =
  ~{name = "What is your name?", hobby = "What is your favourite hobby?"}
  !name(ask)
  !hobby(ask)
  ~"Hello {.name}, I like {.hobby} too!" putLine

ar InputOutput greetIO : {} --> {} =
  io(greet)

Effectful programming will be explained more in the next section. In practice the main points are:

State

In this example we'll define two sorts of effects: integer state and throwing a string error. See here

effect IntState over Base {
  get : {} --> Int,
  put : Int --> {}
}

effect Err over Base {
  err : String --> []
}

This defines a theory IntState for extending the Base category with two distinguished morphisms for state manipulation: get and put. Similarly Err is a theory with a distinguished morphism err : String -> []. Note that [] is the empty sum, i.e. the initial object of Base. Therefore err can be used to map from String to any object, by composing with [], the empty cocone.

We can then define morphisms in this abstract extension of Base. The following morphism increments the state while returning the original value:

ar Base[IntState] next : {} --> Int =
  get ~{ current = , next = incr} !next(put) ~.current

There are two new pieces of syntax:

So next works as follows:

For testing the error effect, we'll make a version which throws an error if the next number is greater than 3:

ar Base[IntState, Err] nextSub3 : {} --> Int =
  next ~( { sub3 = < 3, ok = } @sub3 )
  [ true  = ~.ok,
    false = ~"Was not under 3!" err []]

Next we'll specify how to map this function over a list. We can't reuse the list functor because that doesn't specify how to sequence the effects: should the effect be performed first on the head or the tail of the list?

ar Base[IntState, Err] mapNextSub3 : list({}) --> list(Int) =
    [ empty = ~empty.,
      cons  = !head(nextSub3) !tail(mapNextSub3) ~cons. ]

We explicitly sequence the effects, using composition, on first the head and then the tail of the list.

The effect-category is still abstract, to actually use the above we must define an effect-category over base and interpret the effects:

category ErrIntState {
  ob             = ob Base,
  ar A --> B     = ar Base :
                   { state: Int, value: A } -->
                   [ err: String, suc: { state: Int, value: B } ],
  identity       = suc.,
  f g            = f [ err = err., suc = g ],
  SumOb(idx)     = SumOb(idx),
  sumInj(label)  = { state, value = .value sumInj(label) } suc.,
  sumUni(cocone) = @value sumUni(cocone)
}

This defines a new category with the same objects as Base, but with a different composition, identity and sum. Next we make this into an effect category over Base:

effect_category pureErrIntState ErrIntState over Base {
  ~f      = { state, value = .value f } suc.,
  side(f) =
    { runeff = { state = .state,
                 value = .value .eff } f,
      onside = .value .pur }
    @runeff
    [ err = .runeff err.,
      suc = { state = .runeff .state,
              value = { eff = .runeff .value,
                        pur = .onside } } suc. ]
}

This is done by defining the canoncial injection ~ and side, the action of the effect-category at product components. This is done by interpreting !eff{..}, lifting some effectful morphism A --> B into a product { pur: P, eff: A } --> { pur: P, eff: B }.

Then we provide interpretations for the effects we want to use:

interpret IntState in ErrIntState
  { get = { state = .state, value = .state} suc.,
    put = { state = .value, value = {}    } suc.
  }

interpret Err in ErrIntState
  { err = .value err. }

Finally, we can execute this effect:

ar Base main : {} --> Int =
  { state = 0,                 // initialise the state to 0
    value = #({}, {}, {}) }    // we'll map over a list of size 3
  pureErrIntState(mapNextSub3)

This returns the list #(0, 1, 2), but if one increases the size of the length of the list to 4, then it will instead throw an error.

Checkout the full example.

The Categorical Abstract Machine

Lawvere has a compiler to a Categorical Abstract Machine. Again not all features are supported yet.

Compiling the sum function above (called on an empty list) produces:

$ bill --target vmcode examples/sum.law
#main:
  empty.
  call #sum
#sum:
  cocone empty:#sum_0 cons:#sum_1
#sum_0:
  scal 0
#sum_1:
  push
  scal 1
  push_cone
  pop
  push
  .tail
  call #sum
  push_cone
  pop
  end_cone 1 2
  prim plus

And you can execute the code on the virtual machine with:

bill --target vm examples/sum.law
--------------
Lawvere v0.0.0
--------------
Checking..
Check OK!
Running on categorical machine..
Result: 0

Compiling to JavaScript

To compile to JavaScript, use the --target js option:

$ bill --target js test.law

This will output a JavaScript program that logs the output. You can pipe this directly to node:

$ bill --target js test.law | node
45

The JavaScript compiler isn't well maintained and will just error out on the anything but the most basic language features.

Build/Installation

You can build the project with stack or nix.

Stack

First install stack (curl -sSL https://get.haskellstack.org/ | sh ) and then use stack build. To install the bill executable (to ~/.local/bin) run stack install.

On linux you may also need to install tinfo, e.g. sudo apt-get install libtinfo-dev on Ubuntu.

Nix

Make sure you have nix and optionally direnv installed.

Optional: (but faster) install cachix (nix-env -iA cachix -f https://cachix.org/api/v1/install) and use the lawvere cache: cachix use lawvere.

To build project dependencies and tooling the first time enter a nix shell either using direnv (recommended):

$ echo "use nix" > .envrc
$ direnv allow

or manually:

$ nix-shell

Once in the nix shell, to build a release and run it:

$ nix-build nix/release.nix
$ result/bin/bill <file>

Editor support

For the moment there is only an emacs mode. The syntax looks much better if you use a font with programming ligatures, e.g. FiraCode or alternatives.

Development

To update the nix derivation when project dependencies change:

$ hpack
$ cabal2nix . > nix/packages/lawvere.nix
$ direnv reload

Note: Cabal is also available in the nix shell so you can build with it as well if you like:

$ cabal build

TODO