# DataFlow.jl

DataFlow.jl is a code intermediate representation (IR) format. It can be thought of as antithetical to SSA form; where SSA allows only statements, DataFlow allows only expressions. Closures are represented explicitly, allowing full programs to be easily represented and manipulated and complex whole-program transformations to be applied. Moreover, programs can be kept in a high-level form that's very human-readable.

A data flow graph is a bit like an expression tree without variables; functions always refer to their inputs directly. Underneath it's a directed graph linking the output of one function call to the input of another. DataFlow.jl provides functions like `prewalk`

and `postwalk`

which allow you to do crazy graph-restructuring operations with minimal code, *even on cyclic graphs*. Think algorithms like common subexpression elimination implemented in one line rather than hundreds.

## Basics

```
julia> using DataFlow: vertex, constant, Call
```

DataFlow.jl provides the `IVertex`

data type, which behaves a lot like Julia's `Expr`

type. We can construct vertices, and use them as inputs to other vertices, to build expressions. `constant`

is a shortcut for vertices representing constant values.

```
julia> using DataFlow: vertex, constant, Call, Constant
julia> a, b = constant(1), constant(2)
(IVertex(1), IVertex(2))
julia> c = vertex(Call(), constant(+), a, b)
IVertex((+)(1, 2))
```

The `Call()`

object is analagous to the "head" in Julia's `Expr`

s, so this is like `Expr(:call, f, x...)`

.

A key difference from `Expr`

is that the `IVertex`

is a graph, not a tree, and reuse is explicitly represented. Consider multiplying the expression `c`

by itself:

```
julia> d = vertex(Call(), constant(*), c, c)
IVertex(
bison = (+)(1, 2)
(*)(bison, bison))
```

In order to represent the structure of the graph in text, DataFlow.jl prints an expression tree with a made-up variable binding (`bison`

). This variable is *not* present in the graph itself, but just used for presentation.

Graphs can also be dumped to Julia expressions using `DataFlow.syntax`

, which will similarly create variable bindings where needed.

```
julia> DataFlow.syntax(d)
quote
##edge#668 = (+)(1, 2)
(*)(##edge#668, ##edge#668)
end
julia> eval(ans)
9
```

Graphs are also allowed to be cyclic. We can introduce cycles using `thread!`

, which pushes a new argument into an existing vertex.

```
julia> DataFlow.thread!(c, c)
IVertex(bison = (+)(1, 2, bison))
```

Notice that the cycle is represented by the circular dependency of `bison`

on itself.

## Walking

Transformations are carried out via `prewalk`

and `postwalk`

functions very similar to those in MacroTools (see there for more explanation).

```
julia> DataFlow.prewalk(d) do v
v.value isa Call && v[1].value == Constant(+) ? vertex(constant(-), v[2:end]...) : v
end
IVertex(
bison = (IVertex(-))(1, 2)
(*)(bison, bison))
```

There are also in-place variants of prewalk and postwalk, which can be used for more advanced transformations.