AutoTypeParameters

A Julia library to reversibly encode "any" value so that it can be used as a type parameter.

Do I Need This?

You need this if you have an error like:

ERROR: TypeError: apply_type: in Val, expected Type{T}, got XXX

when you try to create a "complicated" dependent type.

That error is (partly) explained here - it occurs because the kinds of things that can be types in Julia is limited. Partly for sensible reasons (you don't want a mutable type), but also because of arbitrary implementation details.

What Does It Do?

This package has two functions - freeze() and thaw() - that translate arbitrary values back and forth into a form that is accepted by Julia.

How Does It Work?

You can choose between two encondings.

Using showall()

By default, freeze() takes the output from showall() and converts it into a Symbol, while thaw() uses eval() to convert it back into a "real" value:

julia> using AutoTypeParameters

julia> freeze("a string")
symbol("ATP \"a string\"")

julia> thaw(eval, symbol("ATP \"a string\""))
"a string"

The advantage of the default approach is that the type parameter is readable. The disadvantage, of course, is that it requires showall() to generate output that eval() can handle.

Using serialize()

Alternatively (eg for values which do not have a useful showall() function), the base64 encoded output from serialize() can be used:

julia> using AutoTypeParameters

julia> freeze("a string"; format=:serialize)
symbol("ATP=JhWGYSBzdHJpbmc=")

julia> thaw(eval, symbol("ATP=JhWGYSBzdHJpbmc="))
"a string"

Warnings

Because this package uses eval() it should not be passed arbitrary values rom an untrusted user.

Example

julia> using AutoTypeParameters

julia> type MyType{N}
           x
       end

julia> MyType{"strings not allowed"}(42)
ERROR: TypeError: apply_type: in MyType, expected Type{T}, got ASCIIString

julia> MyType(N, x) = MyType{freeze(N)}(x)
MyType{N}

julia> MyType("strings not allowed", 42)
MyType{symbol("ATP \"strings not allowed\"")}(42)

julia> extract_type{N}(x::MyType{N}) = thaw(eval, N)
extract_type (generic function with 1 method)

julia> extract_type(MyType("strings not allowed", 42))
"strings not allowed"