diff --git a/src/Fields/function_field.jl b/src/Fields/function_field.jl
index 41b2c8b5fc..4f2ac384e2 100644
--- a/src/Fields/function_field.jl
+++ b/src/Fields/function_field.jl
@@ -58,9 +58,9 @@ fieldify_function(L, a::Function, grid) = FunctionField(L, a, grid)
 
 Adapt.adapt_structure(to, f::FunctionField{LX, LY, LZ}) where {LX, LY, LZ} =
     FunctionField{LX, LY, LZ}(Adapt.adapt(to, f.func),
-                           Adapt.adapt(to, f.grid),
-                           clock = Adapt.adapt(to, f.clock),
-                           parameters = Adapt.adapt(to, f.parameters))
+                              Adapt.adapt(to, f.grid),
+                              clock = Adapt.adapt(to, f.clock),
+                              parameters = Adapt.adapt(to, f.parameters))
 
 
 on_architecture(to, f::FunctionField{LX, LY, LZ}) where {LX, LY, LZ} =
diff --git a/src/Fields/set!.jl b/src/Fields/set!.jl
index e311f659f1..d1e43a6daa 100644
--- a/src/Fields/set!.jl
+++ b/src/Fields/set!.jl
@@ -43,7 +43,7 @@ end
 ##### Setting to specific things
 #####
 
-function set_to_function!(u, f)
+function set_to_function!(u::Field, f)
     # Supports serial and distributed
     arch = architecture(u)
     child_arch = child_architecture(u)
@@ -53,7 +53,6 @@ function set_to_function!(u, f)
         cpu_arch = cpu_architecture(arch)
         cpu_grid = on_architecture(cpu_arch, u.grid)
         cpu_u    = Field(location(u), cpu_grid; indices = indices(u))
-    
     elseif child_arch isa CPU
         cpu_grid = u.grid
         cpu_u = u
@@ -89,7 +88,7 @@ function set_to_function!(u, f)
     return u
 end
 
-function set_to_array!(u, f)
+function set_to_array!(u::Field, f)
     f = on_architecture(architecture(u), f)
 
     try
@@ -111,7 +110,7 @@ function set_to_array!(u, f)
     return u
 end
 
-function set_to_field!(u, v)
+function set_to_field!(u::Field, v)
     # We implement some niceities in here that attempt to copy halo data,
     # and revert to copying just interior points if that fails.
     
diff --git a/src/OutputReaders/field_time_series.jl b/src/OutputReaders/field_time_series.jl
index 19a1bdb2c8..ea3d7d7201 100644
--- a/src/OutputReaders/field_time_series.jl
+++ b/src/OutputReaders/field_time_series.jl
@@ -23,7 +23,7 @@ using Oceananigans.Utils: launch!
 
 import Oceananigans.Architectures: architecture, on_architecture
 import Oceananigans.BoundaryConditions: fill_halo_regions!, BoundaryCondition, getbc
-import Oceananigans.Fields: Field, set!, interior, indices, interpolate!
+import Oceananigans.Fields: Field, interior, indices, interpolate!
 
 #####
 ##### Data backends for FieldTimeSeries
diff --git a/src/OutputReaders/field_time_series_indexing.jl b/src/OutputReaders/field_time_series_indexing.jl
index d849192f67..e29ab8a404 100644
--- a/src/OutputReaders/field_time_series_indexing.jl
+++ b/src/OutputReaders/field_time_series_indexing.jl
@@ -283,3 +283,4 @@ function getindex(fts::InMemoryFTS, n::Int)
 
     return Field(location(fts), fts.grid; data, fts.boundary_conditions, fts.indices)
 end
+
diff --git a/src/OutputReaders/set_field_time_series.jl b/src/OutputReaders/set_field_time_series.jl
index 577082782e..91bfe7722a 100644
--- a/src/OutputReaders/set_field_time_series.jl
+++ b/src/OutputReaders/set_field_time_series.jl
@@ -1,6 +1,70 @@
 using Printf
 using Oceananigans.Architectures: cpu_architecture
 
+import Oceananigans.Fields: set!
+
+function set!(u::InMemoryFTS, v::InMemoryFTS)
+    if child_architecture(u) === child_architecture(v)
+        # Note: we could try to copy first halo point even when halo
+        # regions are a different size. That's a bit more complicated than
+        # the below so we leave it for the future.
+        
+        try # to copy halo regions along with interior data
+            parent(u) .= parent(v)
+        catch # this could fail if the halo regions are different sizes?
+            # copy just the interior data
+            interior(u) .= interior(v)
+        end
+    else
+        v_data = on_architecture(child_architecture(u), v.data)
+        
+        # As above, we permit ourselves a little ambition and try to copy halo data:
+        try
+            parent(u) .= parent(v_data)
+        catch
+            interior(u) .= interior(v_data, location(v), v.grid, v.indices)
+        end
+    end
+
+    return u
+end
+
+function set!(u::InMemoryFTS, v::Function)
+    # Supports serial and distributed
+    arch = architecture(u)
+    child_arch = child_architecture(u)
+    LX, LY, LZ = location(u)
+
+    # Determine cpu_grid and cpu_u
+    if child_arch isa GPU
+        cpu_arch = cpu_architecture(arch)
+        cpu_grid = on_architecture(cpu_arch, u.grid)
+        cpu_times = on_architecture(cpu_arch, u.times)
+        cpu_u = FieldTimeSeries{LX, LY, LZ}(cpu_grid, cpu_times; indices=indices(u))
+    elseif child_arch isa CPU
+        cpu_arch = child_arch
+        cpu_grid = u.grid
+        cpu_times = u.times
+        cpu_u = u
+    end
+
+    launch!(cpu_arch, cpu_grid, size(cpu_u),
+            _set_fts_to_function!, cpu_u, (LX(), LY(), LZ()), cpu_grid, cpu_times, v)
+
+    # Transfer data to GPU if u is on the GPU
+    child_arch isa GPU && set!(u, cpu_u)
+    
+    return u
+end
+
+@kernel function _set_fts_to_function!(fts, loc, grid, times, func)
+    i, j, k, n = @index(Global, NTuple)
+    X = node(i, j, k, grid, loc...)
+    @inbounds begin
+        fts[i, j, k, n] = func(X..., times[n])
+    end
+end
+
 #####
 ##### set!
 #####
@@ -44,10 +108,10 @@ function set!(fts::InMemoryFTS, path::String=fts.path, name::String=fts.name)
         set!(fts[n], field_n)
     end
 
-    return nothing
+    return fts
 end
 
-set!(fts::InMemoryFTS, value, n::Int) = set!(fts[n], value)
+set!(fts::InMemoryFTS, v, n::Int) = set!(fts[n], value)
 
 function set!(fts::InMemoryFTS, fields_vector::AbstractVector{<:AbstractField})
     raw_data = parent(fts)
@@ -61,7 +125,7 @@ function set!(fts::InMemoryFTS, fields_vector::AbstractVector{<:AbstractField})
 
     close(file)
 
-    return nothing
+    return fts
 end
 
 # Write property only if it does not already exist
@@ -90,6 +154,8 @@ function set!(fts::OnDiskFTS, field::Field, n::Int, time=fts.times[n])
         maybe_write_property!(file, "timeseries/t/$n", time)
         maybe_write_property!(file, "timeseries/$name/$n", Array(parent(field)))
     end
+
+    return fts
 end
 
 function initialize_file!(file, name, fts)
@@ -100,4 +166,5 @@ function initialize_file!(file, name, fts)
     return nothing
 end
 
-set!(fts::OnDiskFTS, path::String, name::String) = nothing
+set!(fts::OnDiskFTS, path::String, name::String) = fts
+
diff --git a/src/OutputWriters/checkpointer.jl b/src/OutputWriters/checkpointer.jl
index d2da594e08..25d93c9925 100644
--- a/src/OutputWriters/checkpointer.jl
+++ b/src/OutputWriters/checkpointer.jl
@@ -190,7 +190,7 @@ end
 ##### set! for checkpointer filepaths
 #####
 
-set!(model, ::Nothing) = nothing
+set!(model::AbstractModel, ::Nothing) = nothing
 
 """
     set!(model, filepath::AbstractString)
@@ -198,7 +198,7 @@ set!(model, ::Nothing) = nothing
 Set data in `model.velocities`, `model.tracers`, `model.timestepper.Gⁿ`, and
 `model.timestepper.G⁻` to checkpointed data stored at `filepath`.
 """
-function set!(model, filepath::AbstractString)
+function set!(model::AbstractModel, filepath::AbstractString)
 
     jldopen(filepath, "r") do file