Optimizer interface and sample implementation (#19)

Reviewed-on: Rubydragon/MetagraphOptimization.jl#19
Co-authored-by: Anton Reinhard <anton.reinhard@proton.me>
Co-committed-by: Anton Reinhard <anton.reinhard@proton.me>

src/task/properties.jl

@@ -30,7 +30,7 @@ compute(t::AbstractDataTask; data...) = data
 
 Fallback implementation of the compute effort of a task, throwing an error.
 """
-function compute_effort(t::AbstractTask)
+function compute_effort(t::AbstractTask)::Float64
     # default implementation using compute
     return error("Need to implement compute_effort()")
 end
@@ -40,7 +40,7 @@ end
 
 Fallback implementation of the data of a task, throwing an error.
 """
-function data(t::AbstractTask)
+function data(t::AbstractTask)::Float64
     return error("Need to implement data()")
 end
 
@@ -49,28 +49,28 @@ end
 
 Return the compute effort of a data task, always zero, regardless of the specific task.
 """
-compute_effort(t::AbstractDataTask) = 0.0
+compute_effort(t::AbstractDataTask)::Float64 = 0.0
 
 """
     data(t::AbstractDataTask)
 
 Return the data of a data task. Given by the task's `.data` field.
 """
-data(t::AbstractDataTask) = getfield(t, :data)
+data(t::AbstractDataTask)::Float64 = getfield(t, :data)
 
 """
     data(t::AbstractComputeTask)
 
 Return the data of a compute task, always zero, regardless of the specific task.
 """
-data(t::AbstractComputeTask) = 0.0
+data(t::AbstractComputeTask)::Float64 = 0.0
 
 """
     compute_effort(t::FusedComputeTask)
 
 Return the compute effort of a fused compute task. 
 """
-function compute_effort(t::FusedComputeTask)
+function compute_effort(t::FusedComputeTask)::Float64
     return compute_effort(t.first_task) + compute_effort(t.second_task)
 end
 
@@ -79,4 +79,4 @@ end
 
 Return a tuple of a the fused compute task's components' types.
 """
-get_types(::FusedComputeTask{T1, T2}) where {T1, T2} = (T1, T2)
+get_types(t::FusedComputeTask) = (typeof(t.first_task), typeof(t.second_task))