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[sc-630] Update the readme for Bend
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README.md
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README.md
@ -1,20 +1,20 @@
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# HVM-Lang
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# Bend
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HVM-Lang is a lambda-calculus based language and serves as an Intermediate Representation for HVM-Core, offering a higher level syntax for writing programs based on the [Interaction-Calculus](https://github.com/VictorTaelin/Interaction-Calculus#interaction-calculus).
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Bend is a programming language that can run massively parallel programs on the GPU or the CPU using the power of interaction nets and the [HVM](https://github.com/HigherOrderCO/hvm2).
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With Bend, you can write programs for the GPU as easily as you'd write a normal program in your favorite language.
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Compilers that want to target the HVM should compile the source language to HVM-lang, which takes care of converting interaction calculus into the underlying interaction networks.
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It is based on the [Interaction-Calculus](https://github.com/VictorTaelin/Interaction-Calculus#interaction-calculus), a variation of the untyped lambda calculus that compiles efficiently to interaction nets.
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Note that HVM-lang is untyped and does not guarantee correctness or soundness. Compilers that don't want to implement the necessary check can instead transpile to the [Kind language](https://github.com/HigherOrderCO/kind2), which compiles to HVM-lang and implements type-level checking.
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Currently Bend only supports strict/eager evaluation. If you need lazy, optimal evaluation, we recommend using [HVM1](https://github.com/HigherOrderCO/HVM) for now.
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Programmers looking for an HVM-based programming language should also use Kind, which is designed to be user-interfacing.
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## Installation
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With the nightly version of rust installed, clone the repository:
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```bash
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git clone https://github.com/HigherOrderCO/hvm-lang.git
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git clone https://github.com/HigherOrderCO/bend.git
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cd hvm-lang
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cd bend
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```
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Install using cargo:
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@ -22,161 +22,208 @@ Install using cargo:
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cargo install --path . --locked
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```
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If you want to run programs directly from Bend, you also need to have [HVM](https://github.com/HigherOrderCO/hvm2) installed.
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## Usage
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First things first, let's write a basic program that adds the numbers 3 and 2.
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Command | Usage | Description
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--------- | --------------------- | ---
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Check | `bend check <file>` | Checks if a program is valid
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Compile | `bend compile <file>` | Compiles a program to HVM and outputs it to stdout
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Run | `bend run <file>` | Compiles and then runs a program in HVM
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Normalize | `bend norm <file>` | Compiles and then normalizes a program in HVM, outputting the result to stdout
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Desugar | `bend desugar <file>` | Desugars a program to the core syntax and outputs it to stdout
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```hs
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main = (+ 3 2)
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```
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HVM-Lang searches for the `main | Main` definitions as entrypoint of the program.
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To run a program, use the `run` argument:
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If you want to compile a file to a file, just redirect the output with `>`:
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```bash
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hvml run <file>
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bend compile <file.bend> > <file.hvm>
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```
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It will show the number 5.
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Adding the `--stats` option displays some runtime stats like time and rewrites.
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There are many compiler options that can be passed through the CLI. You can see the list of options [here](docs/compiler-options.md).
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To limit the runtime memory, use the `--mem <size> option.` The default is 1GB:
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```bash
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hvml --mem 65536 run <file>
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## Examples
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Bend offers two flavors of syntax, a user-friendly python-like syntax (the default) and the core ML/Haskell-like syntax that's used internally by the compiler.
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You can read the full reference for both of them [here](docs/syntax.md), but these examples will use the first one.
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To see some more complex examples programs, check out the [examples](examples/) folder.
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We can start with a basic program that adds the numbers 3 and 2.
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```py
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def main:
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return 2 + 3
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```
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You can specify the memory size in bytes (default), kilobytes (k), megabytes (m), or gigabytes (g), e.g., `--mem 200m.`
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Normalizing this program will show the number 5.
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Be careful with `run` and `norm`, since they will not show any warnings by default. Before running a new program it's useful to first `check` it.
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To compile a program use the `compile` argument:
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```bash
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hvml compile <file>
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```
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This will output the compiled file to stdout.
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Bend programs consist of a series of function definitions, always starting with a function called `main` or `Main`.
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There are compiler options through the CLI. [Click here](docs/compiler-options.md) to learn about them.
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Functions can receive arguments both directly and using a lambda abstraction.
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```py
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// These two are equivalent
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def add(x, y):
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return x + y
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## Syntax
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HVM-Lang files consists of a series of definitions, which bind a name to a term. Terms include lambda-calculus abstractions and applications, numbers, tuples, among others.
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Here's a lambda where the body is the variable `x`:
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```rs
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id = λx x
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def add2:
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return lambda x, y: x + y
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```
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Lambdas can also be defined using `@`.
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To discard the variable and not bind it to any name, use `*`:
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```hs
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True = @t @* t
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False = λ* λf f
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You can then call this function like this:
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```py
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def main:
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sum = add(2, 3)
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return sum
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```
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Applications are enclosed by `(` `)`.
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```rs
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(λx x λx x λx x)
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```
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This term is the same as:
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```rs
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(((λx x) (λx x)) (λx x))
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```
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Parentheses around lambdas are optional. Lambdas have a high precedence
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You can bundle multiple values into a single value using a tuple or a struct.
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```py
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// With a tuple
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def Tuple.fst(x):
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// This destructures the tuple into the two values it holds.
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// '*' means that the value is discarded and not bound to any variable.
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(fst, *) = x
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return fst
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```rust
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(λx a b) == ((λx a) b) != (λx (a b))
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// With a struct
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struct Pair(fst, snd):
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def Pair.fst(x):
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match x:
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Pair:
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return x.fst
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// We can also directly access the fields of a struct.
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// This requires that we tell the compiler the type of the variable where it is defined.
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def Pair.fst_2(x: Pair):
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return x.fst
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```
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`*` can also be used to define an eraser term.
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It compiles to an `inet` node with only one port that deletes anything that’s plugged into it.
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```rs
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era = *
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For more complicated data structures, we can use `enum` to define a algebraic data types.
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```py
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enum MyTree:
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Node(val, ~left, ~right)
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Leaf
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```
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A let term binds some value to the next term, in this case `(* result 2)`:
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```rs
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let result = (+ 1 2); (* result 2)
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We can then pattern match on the enum to perform different actions depending on the variant of the value.
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```py
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def Maybe.or_default(x, default):
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match x:
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Maybe/some:
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// We can access the fields of the variant using 'matched.field'
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return x.val
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Maybe/none:
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return default
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```
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The `use` term inlines clones of some value to the next term:
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```rs
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use result = (+ 1 2); (* result result)
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We use `~` to indicate that a field is recursive.
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This allows us to easily create and consume these recursive data structures with `bend` and `fold`:
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```py
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def MyTree.sum(x):
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// Sum all the values in the tree.
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fold x:
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// The fold is implicitly called for fields marked with '~' in their definition.
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Node:
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return val + x.left + x.right
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Leaf:
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return 0
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// Equivalent to
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(* (+ 1 2) (+ 1 2))
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```
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The same term with `let` duplicates the value:
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```rs
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let result = (+ 1 2); (* result result)
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def main:
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bend val = 0 while val < 0:
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// 'go' calls the bend recursively with the provided values.
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x = Node(val=val, left=go(val + 1), right=go(val + 1))
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then:
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// 'then' is the base case, when the condition fails.
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x = Leaf
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// Equivalent to
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let {result_1 result_2} = (+ 1 2); (* result_1 result_2)
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return MyTree.sum(x)
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```
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These are equivalent to inline recursive functions that create a tree and consume it.
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```py
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def MyTree.sum(x):
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match x:
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Node:
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return x.val + MyTree.sum(x.left) + MyTree.sum(x.right)
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Leaf:
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return 0
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def main_bend(val):
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if val < 0:
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return Node(val, main_bend(val + 1), main_bend(val + 1))
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else:
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return Leaf
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def main:
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return main_bend(0)
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```
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Making your program around trees is a very good way of making it parallelizable, since each core can be dispatched to work on a different branch of the tree.
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*Attention*: Note that despite the ADT syntax sugars, Bend is an *untyped* language and the compiler will not stop you from using values incorrectly, which can lead to very unexpected results.
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For example, the following program will compile just fine even though `!=` is only defined for native numbers:
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```py
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def main:
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bend val = [0, 1, 2, 3] while val != []:
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match val:
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List.cons:
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x = val.head + go(val.tail)
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List.nil:
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x = 0
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then:
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x = 0
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return x
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```
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Normalizing this program will show `λ* *` and not the expected `6`.
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It's also important to note that Bend is linear (technically affine), meaning that every variable is only used once. When a variable is used more than once, the compiler will automatically insert a duplication.
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Duplications efficiently share the same value between two locations, only cloning a value when it's actually needed, but their exact behaviour is slightly more complicated than that and escapes normal lambda-calculus rules.
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You can read more about it in [Dups and sups](docs/dups-and-sups.md).
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You can read more about it in [Dups and sups](docs/dups-and-sups.md) and learn how pattern matching avoids this problem in [Pattern matching](docs/pattern-matching.md).
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It is possible to define tuples:
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```rs
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tup = (2, 2)
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To use a variable twice without duplicating it, you can use a `use` statement.
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It inlines clones of some value in the statements that follow it.
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```py
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def foo(x):
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use result = bar(1, x)
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return (result, result)
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// Is equivalent to
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def foo(x):
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return (bar(1, x), bar(1, x))
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```
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Note that any variable in the `use` will end up being duplicated.
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Bend supports recursive functions of unrestricted depth:
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```py
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def native_num_to_adt(n):
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if n == 0:
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return Nat.zero
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else:
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return Nat.succ(native_num_to_adt(n - 1))
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```
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If your recursive function is not based on pattern matching syntax (like `if`, `match`, `fold`, etc) you have to be careful to avoid an infinite loop.
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Since Bend is eagerly executed, some situations will cause function applications to always be expanded, which can lead to looping situations.
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You can read how to avoid this in [Lazy definitions](docs/lazy-definitions.md).
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Bend has native numbers and operations.
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```py
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def main:
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a = 1 // A 24 bit unsigned integer.
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b = +2 // A 24 bit signed integer.
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c = -3 // Another signed integer, but with negative value.
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d = 1.0 // A 24 bit floating point number.
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e = +0.001 // Also a float.
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return (a * 2, b - c, d / e)
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```
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And destructuring tuples with `let`:
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```rs
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let (x, y) = tup; (+ x y)
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```
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Strings are delimited by `"` `"` and support Unicode characters.
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```rs
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main = "Hello, 🌎"
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```
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A string is desugared to a String data type containing two constructors, `String.cons` and `String.nil`.
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```rs
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// These two are equivalent
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StrEx1 = "Hello"
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data String = (String.cons head tail) | String.nil
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StrEx2 = (String.cons 'H' (String.cons 'e', (String.cons 'l' (String.cons 'l', (String.cons 'o' String.nil)))))
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```
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Characters are delimited by `'` `'` and support Unicode escape sequences. They have a numeric value associated with them.
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```
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main = '\u{4242}'
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```
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Lists are delimited by `[` `]` and elements can be optionally separated by `,`.
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```rs
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ids = [3, 6, 9 12 16]
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```
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A list is desugared to a List data type containing two constructors, `List.cons` and `List.nil`.
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```rs
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// These two are equivalent
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ListEx1 = [1, 2, 3]
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data List = (List.cons head tail) | (List.nil)
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ListEx2 = (List.cons 1 (List.cons 2 (List.cons 3 List.nil)))
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```
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Hvm-lang supports pattern matching through `match` and `switch` terms.
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`match` pattern matches on constructors declared with `data`.
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```rs
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data Option = (Some val) | None
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// 'match' implicitly binds a variable for each field in the constructor.
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// The name of the bound variable depends on the name of the argument.
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map f x = match x {
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Some: (Some (f x.val))
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None: None
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}
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// You can give a name to the match argument to access its fields.
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TakeErr fallible_fn x errs =
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match res = (fallible_fn x) {
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// We can now access res.val.
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// If no name is given, it will be inaccessible.
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Result.ok: ((Some res.val), errs)
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Result.err: (None, (List.cons res.val errs))
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}
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```
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`switch` pattern matches on native numbers:
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```rs
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match x = 4 {
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`switch` pattern matches on unsigned native numbers:
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```py
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switch x = 4:
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// From '0' to n, ending with the default case '_'.
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0: "zero"
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1: "one"
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// The default case binds the name <arg>-<n>
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// where 'arg' is the name of the argument and 'n' is the next number.
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// In this case, it's 'x-3', which will have value (4 - 3) = 1
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_: (String.concat "other: " (String.from_num x-3))
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}
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_: String.concat("other: ", (String.from_num x-3))
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```
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Which is the equivalent of nesting match terms:
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Bend has Lists and Strings, which support Unicode characters.
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```rs
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match x = 4 {
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0: "zero"
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_: match x-1 {
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0: "one"
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_: use x-2 = x-1-1; match x-2 {
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0: "two"
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_: use x-3 = x-2-1; (String.concat "other: " (String.from_num x-3))
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}
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}
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}
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def main:
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return ["You: Hello, 🌎", "🌎: Hello, user"]
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```
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A string is desugared to a String data type containing two constructors, `String.cons` and `String.nil`.
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List also becomes a type with two constructors, `List.cons` and `List.nil`.
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```rs
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// These two are equivalent
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def StrEx:
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"Hello"
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def ids:
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[1, 2, 3]
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// These types are builtin.
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enum String:
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String.cons(head, tail)
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String.nil
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enum List:
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List.cons(head, tail)
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List.nil
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def StrEx:
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String.cons('H', String.cons('e', String.cons('l', String.cons('l', String.cons('o', String.nil)))))
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def ids:
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List.cons(1, List.cons(2, List.cons(3, List.nil)))
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```
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Characters are delimited by `'` `'` and support Unicode escape sequences. They are encoded as a U24 with the unicode codepoint as their value.
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```
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// These two are equivalent
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def chars:
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['A', '\u{4242}', '🌎']
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def chars2:
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[65, 0x4242, 0x1F30E]
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```
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### More features
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@ -219,7 +290,6 @@ Other features are described in the following documentation files:
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- 📗 CLI arguments: [CLI arguments](docs/cli-arguments.md)
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- 📙 Duplications and superpositions: [Dups and sups](docs/dups-and-sups.md)
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- 📙 Scopeless lambdas: [Using scopeless lambdas](docs/using-scopeless-lambdas.md)
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- 📙 Tagged lambdas and applications: [Automatic vectorization with tagged lambdas](docs/automatic-vectorization-with-tagged-lambdas.md)
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- 📕: Fusing functions: [Writing fusing functions](docs/writing-fusing-functions.md)
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## Further reading
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