Julia Language Metaprogramming Python `dict`/JSON like syntax for `Dict` literals.
Example
Introduction
Julia uses the following syntax for dictionaries:
Dict({k₁ => v₁, k₂ => v₂, …, kₙ₋₁ => vₙ₋₁, kₙ => vₙ)
While Python and JSON looks like this:
{k₁: v₁, k₂: v₂, …, kₙ₋₁: vₙ₋₁, kₙ: vₙ}
For illustrative purposes we could also use this syntax in Julia and add new semantics to it (Dict syntax is the idiomatic way in Julia, which is recommended).
First let's see what kind of expression it is:
julia> parse("{1:2 , 3: 4}") |> Meta.show_sexpr
(:cell1d, (:(:), 1, 2), (:(:), 3, 4))
This means we need to take this :cell1d expression and either transform it or return a new expression that should look like this:
julia> parse("Dict(1 => 2 , 3 => 4)") |> Meta.show_sexpr
(:call, :Dict, (:(=>), 1, 2), (:(=>), 3, 4))
Macro definition
The following macro, while simple, allows to demonstrate such code generation and transformation:
macro dict(expr)
# Check the expression has the correct form:
if expr.head ≠ :cell1d || any(sub_expr.head ≠ :(:) for sub_expr ∈ expr.args)
error("syntax: expected `{k₁: v₁, k₂: v₂, …, kₙ₋₁: vₙ₋₁, kₙ: vₙ}`")
end
# Create empty `:Dict` expression which will be returned:
block = Expr(:call, :Dict) # :(Dict())
# Append `(key => value)` pairs to the block:
for pair in expr.args
k, v = pair.args
push!(block.args, :($k => $v))
end # :(Dict(k₁ => v₁, k₂ => v₂, …, kₙ₋₁ => vₙ₋₁, kₙ => vₙ))
# Block is escaped so it can reach variables from it's calling scope:
return esc(block)
end
Let's check out the resulting macro expansion:
julia> :(@dict {"a": :b, 'c': 1, :d: 2.0}) |> macroexpand
:(Dict("a" => :b,'c' => 1,:d => 2.0))
Usage
julia> @dict {"a": :b, 'c': 1, :d: 2.0}
Dict{Any,Any} with 3 entries:
"a" => :b
:d => 2.0
'c' => 1
julia> @dict {
"string": :b,
'c' : 1,
:symbol : π,
Function: print,
(1:10) : range(1, 10)
}
Dict{Any,Any} with 5 entries:
1:10 => 1:10
Function => print
"string" => :b
:symbol => π = 3.1415926535897...
'c' => 1
The last example is exactly equivalent to:
Dict(
"string" => :b,
'c' => 1,
:symbol => π,
Function => print,
(1:10) => range(1, 10)
)
Misusage
julia> @dict {"one": 1, "two": 2, "three": 3, "four": 4, "five" => 5}
syntax: expected `{k₁: v₁, k₂: v₂, …, kₙ₋₁: vₙ₋₁, kₙ: vₙ}`
julia> @dict ["one": 1, "two": 2, "three": 3, "four": 4, "five" => 5]
syntax: expected `{k₁: v₁, k₂: v₂, …, kₙ₋₁: vₙ₋₁, kₙ: vₙ}`
Notice that Julia has other uses for colon : as such you will need to wrap range literal expressions with parenthesis or use the range function, for example.
