[julia-users] Computed type for parametrized method definition

[Benjamin Piwowarski]

Hi,

I would like to use a computed type for a parametrized method definition, 
in order to catch as early as possible errors in types. I tried this

abstract typeA

immutable typeA1 <: typeA end
immutable typeA2 <: typeA end

type typeB1 end
type typeB2 end

function transformType(T)
    if T == typeA1
       return typeB1
    elseif T == typeA2
       return typeB2
    end
    error("Cannot handle type $T")
end

function compute{T<:typeA}(a::transformType(T), b::transformType(T))
    return (a, b)
end

but it failed with the following error message

ERROR: Cannot handle type T<:typeA
 in error at error.jl:21
 in transformType at /Users/bpiwowar/Desktop/test.jl:15
 in include at 
/opt/homebrew-cask/Caskroom/julia/0.3.5/Julia-0.3.5.app/Contents/Resources/julia/lib/julia/sys.dylib
 in include_from_node1 at loading.jl:128
 in process_options at 
/opt/homebrew-cask/Caskroom/julia/0.3.5/Julia-0.3.5.app/Contents/Resources/julia/lib/julia/sys.dylib
 in _start at 
/opt/homebrew-cask/Caskroom/julia/0.3.5/Julia-0.3.5.app/Contents/Resources/julia/lib/julia/sys.dylib
while loading /Users/bpiwowar/Desktop/test.jl, in expression starting on 
line 18

The type transformType(T)is hence computed when the method is defined, but 
not when the method is called. Is there any way to prevent this (to defer 
evaluation at compilation time?). 

The only other way (I can think of) to get this result would be to use 
macros and enumerate the different possible types, but if somebody can 
think of a better solution, thanks!