Fix for default arguments (#474)

Currently on master:

``` julia
julia> using DynamicPPL, Distributions, LinearAlgebra

julia> @model test_defaults(x, n=length(x)) = x ~ MvNormal(zeros(n), I)
test_defaults (generic function with 3 methods)

julia> test_defaults(missing, 2)()
ERROR: MethodError: no method matching length(::Missing)
...
```

On this branch:

``` julia
julia> using DynamicPPL, Distributions, LinearAlgebra

julia> @model test_defaults(x, n=length(x)) = x ~ MvNormal(zeros(n), I)
test_defaults (generic function with 3 methods)

julia> test_defaults(missing, 2)()
2-element Vector{Float64}:
 0.3028550279042859
 0.6130034982853375
```

On master, this is caused by

``` julia
julia> @macroexpand @model test_defaults(x, n=length(x)) = x ~ MvNormal(zeros(n), I)
quote
    function test_defaults(__model__::Model, __varinfo__::AbstractVarInfo, __context__::AbstractPPL.AbstractContext, x, n; )
        ...
    end
    begin
        $(Expr(:meta, :doc))
        function test_defaults(x, n = length(x); )
            # This is the offending line.
            return (Model)(test_defaults, NamedTuple{(:x, :n)}((x, n)), NamedTuple{(:n,)}((length(x),)))
        end
    end
end
```

And subsequently fixed in this PR, in which case the above is

```julia
julia> @macroexpand @model test_defaults(x, n=length(x)) = x ~ MvNormal(zeros(n), I)
quote
    function test_defaults(__model__::Model, __varinfo__::AbstractVarInfo, __context__::AbstractPPL.AbstractContext, x, n; )
        ...
    end
    begin
        $(Expr(:meta, :doc))
        function test_defaults(x, n = length(x); )
            # Now we respect the argument `n`.
            return (Model)(test_defaults, NamedTuple{(:x, :n)}((x, n)), NamedTuple{(:n,)}((n,)))
        end
    end
end
```


Co-authored-by: Hong Ge <[email protected]>

Author: torfjelde

Project.toml

@@ -1,6 +1,6 @@
 name = "DynamicPPL"
 uuid = "366bfd00-2699-11ea-058f-f148b4cae6d8"
-version = "0.22.2"
+version = "0.22.3"
 
 [deps]
 AbstractMCMC = "80f14c24-f653-4e6a-9b94-39d6b0f70001"

src/compiler.jl

@@ -287,7 +287,7 @@ function build_model_info(input_expr)
     end
 
     # Build named tuple expression of the argument symbols with default values.
-    defaults_namedtuple = to_namedtuple_expr(default_syms, default_vals)
+    defaults_namedtuple = to_namedtuple_expr(default_syms)
 
     modelinfo = Dict(
         :allargs_exprs => allargs_exprs,

src/utils.jl

@@ -161,7 +161,15 @@ function getargs_assignment(expr::Expr)
     end
 end
 
-function to_namedtuple_expr(syms, vals=syms)
+function to_namedtuple_expr(syms)
+    length(syms) == 0 && return :(NamedTuple())
+
+    names_expr = Expr(:tuple, QuoteNode.(syms)...)
+    return :(NamedTuple{$names_expr}(($(syms...),)))
+end
+
+# FIXME: the prob macro still uses this.
+function to_namedtuple_expr(syms, vals)
     length(syms) == 0 && return :(NamedTuple())
 
     names_expr = Expr(:tuple, QuoteNode.(syms)...)

test/model.jl

@@ -149,4 +149,9 @@ end
         Random.seed!(1776)
         @test rand(Dict, model) == sample_dict
     end
+
+    @testset "default arguments" begin
+        @model test_defaults(x, n=length(x)) = x ~ MvNormal(zeros(n), I)
+        @test length(test_defaults(missing, 2)()) == 2
+    end
 end