Readme · JET.jl

behind the moar for performance ...

JET.jl employs Julia's type inference for bug reports.

!!! note JET.jl needs Julia versions 1.6 and higher; JET is tested against the current stable release and an nightly version.
Also note that JET deeply relies on the type inference routine implemented in Julia's compiler, and so JET analysis result can vary depending on your Julia version. In general, the newer your Julia version is, more accurately and quickly your can expect JET to analyze your code, assuming Julia's compiler keeps evolving all the time from now on.

Documentation

JET's documentation is now available at here ! Any kind of feedback or help will be very appreciated.

Demo

Say you have this strange and buggy file and want to know where to fix:

demo.jl

# demo
# ====

# fibonacci
# ---------

fib(n) = n ≤ 2 ? n : fib(n-1) + fib(n-2)

fib(1000)   # never terminates in ordinal execution
fib(m)      # undef var
fib("1000") # obvious type error


# language features
# -----------------

# user-defined types, macros
struct Ty{T}
    fld::T
end

function foo(a)
    v = Ty(a)
    return bar(v)
end

# macros will be expanded
@inline bar(n::T)     where {T<:Number} = n < 0 ? zero(T) : one(T)
@inline bar(v::Ty{T}) where {T<:Number} = bar(v.fdl) # typo "fdl"
@inline bar(v::Ty)                      = bar(convert(Number, v.fld))

foo(1.2)
foo("1") # `String` can't be converted to `Number`

# even staged programming
# adapted from https://github.com/JuliaLang/julia/blob/9f665c19e076ab37cbca2d0cc99283b82e99c26f/base/namedtuple.jl#L253-L264
@generated function badmerge(a::NamedTuple{an}, b::NamedTuple{bn}) where {an, bn}
    names = Base.merge_names(an, bn)
    types = Base.merge_types(names, a, b)
    vals = Any[ :(getfield($(Base.sym_in(names[n], bn) ? :b : :a), $(names[n]))) for n in 1:length(names) ] # missing quote, just ends up with under vars
    :( NamedTuple{$names,$types}(($(vals...),)) )
end

badmerge((x=1,y=2), (y=3,z=1))

You can have JET.jl detect possible errors:

julia> using JET

julia> report_and_watch_file("demo.jl"; annotate_types = true)
[toplevel-info] virtualized the context of Main (took 0.013 sec)
[toplevel-info] entered into demo.jl
[toplevel-info]  exited from demo.jl (took 3.254 sec)
═════ 7 possible errors found ═════
┌ @ demo.jl:10 fib(m)
│ variable m is not defined: fib(m)
└──────────────
┌ @ demo.jl:11 fib("1000")
│┌ @ demo.jl:7 ≤(n::String, 2)
││┌ @ operators.jl:401 Base.<(x::String, y::Int64)
│││┌ @ operators.jl:352 Base.isless(x::String, y::Int64)
││││ no matching method found for call signature (Tuple{typeof(isless), String, Int64}): Base.isless(x::String, y::Int64)
│││└────────────────────
┌ @ demo.jl:32 foo(1.2)
│┌ @ demo.jl:24 bar(v::Ty{Float64})
││┌ @ demo.jl:29 Base.getproperty(v::Ty{Float64}, :fdl::Symbol)
│││┌ @ Base.jl:33 Base.getfield(x::Ty{Float64}, f::Symbol)
││││ type Ty{Float64} has no field fdl
│││└──────────────
┌ @ demo.jl:33 foo("1")
│┌ @ demo.jl:24 bar(v::Ty{String})
││┌ @ demo.jl:30 convert(Number, Base.getproperty(v::Ty{String}, :fld::Symbol)::String)
│││ no matching method found for call signature (Tuple{typeof(convert), Type{Number}, String}): convert(Number, Base.getproperty(v::Ty{String}, :fld::Symbol)::String)
││└──────────────
┌ @ demo.jl:44 badmerge(Core.apply_type(Core.NamedTuple, Core.tuple(:x::Symbol, :y::Symbol)::Tuple{Symbol, Symbol})::Type{NamedTuple{(:x, :y)}}(Core.tuple(1, 2)::Tuple{Int64, Int64})::NamedTuple{(:x, :y), Tuple{Int64, Int64}}, Core.apply_type(Core.NamedTuple, Core.tuple(:y::Symbol, :z::Symbol)::Tuple{Symbol, Symbol})::Type{NamedTuple{(:y, :z)}}(Core.tuple(3, 1)::Tuple{Int64, Int64})::NamedTuple{(:y, :z), Tuple{Int64, Int64}})
│┌ @ demo.jl:37 getfield(a::NamedTuple{(:x, :y), Tuple{Int64, Int64}}, x)
││ variable x is not defined: getfield(a::NamedTuple{(:x, :y), Tuple{Int64, Int64}}, x)
│└──────────────
│┌ @ demo.jl:37 getfield(b::NamedTuple{(:y, :z), Tuple{Int64, Int64}}, y)
││ variable y is not defined: getfield(b::NamedTuple{(:y, :z), Tuple{Int64, Int64}}, y)
│└──────────────
│┌ @ demo.jl:37 getfield(b::NamedTuple{(:y, :z), Tuple{Int64, Int64}}, z)
││ variable z is not defined: getfield(b::NamedTuple{(:y, :z), Tuple{Int64, Int64}}, z)
│└──────────────

Hooray ! JET.jl found possible error points (e.g. MethodError: no method matching isless(::String, ::Int64)) given toplevel call signatures of generic functions (e.g. fib("1000")).

Note that JET can find these errors while demo.jl is so inefficient (especially the fib implementation) that it would never terminate in actual execution. That is possible because JET analyzes code only on type level. This technique is often called "abstract interpretation" and JET internally uses Julia's native type inference implementation, so it can analyze code as fast/correctly as Julia's code generation.

Lastly let's apply the following diff to demo.jl so that it works nicely:

fix-demo.jl.diff

diff --git a/demo.jl b/demo.jl
index f868d2f..634e130 100644
--- a/demo.jl
+++ b/demo.jl
@@ -4,11 +4,21 @@
 # fibonacci
 # ---------

-fib(n) = n ≤ 2 ? n : fib(n-1) + fib(n-2)
+# cache, cache, cache
+function fib(n::T) where {T<:Number}
+    cache = Dict(zero(T)=>zero(T), one(T)=>one(T))
+    return _fib(n, cache)
+end
+_fib(n, cache) = if haskey(cache, n)
+    cache[n]
+else
+    cache[n] = _fib(n-1, cache) + _fib(n-2, cache)
+end

-fib(1000)   # never terminates in ordinal execution
-fib(m)      # undef var
-fib("1000") # obvious type error
+fib(BigInt(1000)) # will terminate in ordinal execution as well
+m = 1000          # define m
+fib(m)
+fib(parse(Int, "1000"))


 # language features
@@ -26,19 +36,19 @@ end

 # macros will be expanded
 @inline bar(n::T)     where {T<:Number} = n < 0 ? zero(T) : one(T)
-@inline bar(v::Ty{T}) where {T<:Number} = bar(v.fdl) # typo "fdl"
+@inline bar(v::Ty{T}) where {T<:Number} = bar(v.fld) # typo fixed
 @inline bar(v::Ty)                      = bar(convert(Number, v.fld))

 foo(1.2)
-foo("1") # `String` can't be converted to `Number`
+foo('1') # `Char` will be converted to `UInt32`

 # even staged programming
 # adapted from https://github.com/JuliaLang/julia/blob/9f665c19e076ab37cbca2d0cc99283b82e99c26f/base/namedtuple.jl#L253-L264
-@generated function badmerge(a::NamedTuple{an}, b::NamedTuple{bn}) where {an, bn}
+@generated function goodmerge(a::NamedTuple{an}, b::NamedTuple{bn}) where {an, bn}
     names = Base.merge_names(an, bn)
     types = Base.merge_types(names, a, b)
-    vals = Any[ :(getfield($(Base.sym_in(names[n], bn) ? :b : :a), $(names[n]))) for n in 1:length(names) ] # missing quote, just ends up with under vars
+    vals = Any[ :(getfield($(Base.sym_in(names[n], bn) ? :b : :a), $(QuoteNode(names[n])))) for n in 1:length(names) ] # names quoted, should work as expected
     :( NamedTuple{$names,$types}(($(vals...),)) )
 end

-badmerge((x=1,y=2), (y=3,z=1))
+goodmerge((x=1,y=2), (y=3,z=1))

If you apply the diff (i.e. update and save the demo.jl), JET will automatically re-trigger analysis, and this time, won't complain anything:

git apply fix-demo.jl.diff

[toplevel-info] virtualized the context of Main (took 0.004 sec)
[toplevel-info] entered into demo.jl
[toplevel-info]  exited from demo.jl (took 3.061 sec)
No errors !

Roadmap

editor/IDE integration: GUI would definitely be more appropriate for showing JET's analysis result
- smarter code dependency tracking: the watch mode currently re-analyzes the whole code on each update, which is the most robust and least efficient option. When integrated with an IDE, fancier incremental analysis based on smarter code dependency tracking like what Revise.jl does would be needed
- LSP support: ideally I hope to extend JET to provide some of LSP features other than diagnostics, e.g. auto-completions, rename refactor, taking type-level information into account
more documentation: especially JET needs a specification of its error reports
more accurate and faster analysis: abstract interpretation can be improved yet more, e.g. with alias analysis
configurable strictness: there're cases where more strict check is appropriate, and vice versa
performance linting: JET can be used to report performance pitfalls, e.g. report a dynamic dispatch found inside a heavy loop

Acknowledgement

This project started as my grad thesis project at Kyoto University, supervised by Prof. Takashi Sakuragawa. We were heavily inspired by ruby/typeprof, an experimental type understanding/checking tool for Ruby. The grad thesis about this project is published at https://github.com/aviatesk/grad-thesis, but currently it's only available in Japanese.