https://github.com/akkartik/mu/blob/master/044space_surround.cc

     1 //: So far you can have global variables by not setting default-space, and
     2 //: local variables by setting default-space. You can isolate variables
     3 //: between those extremes by creating 'surrounding' spaces.
     4 //:
     5 //: (Surrounding spaces are like lexical scopes in other languages.)
     6
     7 :(scenario surrounding_space)
     8 # location 2 in space 1 (remember that locations 0 and 1 are reserved in all
     9 # spaces) refers to the space surrounding the default space, here 20.
    10 def main [
    11   # prepare default-space address
    12   10:num/alloc-id, 11:num <- copy 0, 1000
    13   # prepare default-space payload
    14   1000:num <- copy 0  # alloc id of payload
    15   1001:num <- copy 5  # length
    16   # prepare address of chained space
    17   20:num/alloc-id, 21:num <- copy 0, 2000
    18   # prepare payload of chained space
    19   2000:num <- copy 0  # alloc id of payload
    20   2001:num <- copy 5  # length
    21   # actual start of this recipe
    22   default-space:space <- copy 10:space
    23   #: later layers will explain the /names: property
    24   0:space/names:dummy <- copy 20:space/raw
    25   2:num <- copy 94
    26   2:num/space:1 <- copy 95
    27 ]
    28 def dummy [  # just for the /names: property above
    29 ]
    30 # chain space: 1000 + (alloc id) 1 + (length) 1
    31 +mem: storing 0 in location 1002
    32 +mem: storing 2000 in location 1003
    33 # store to default space: 1000 + (alloc id) 1 + (length) 1 + (index) 2
    34 +mem: storing 94 in location 1004
    35 # store to chained space: (contents of location 1003) 2000 + (alloc id) 1 + (length) 1 + (index) 2
    36 +mem: storing 95 in location 2004
    37
    38 //: If you think of a space as a collection of variables with a common
    39 //: lifetime, surrounding allows managing shorter lifetimes inside a longer
    40 //: one.
    41
    42 :(replace{} "int space_base(const reagent& x)")
    43 int space_base(const reagent& x) {
    44   int base = current_call().default_space ? (current_call().default_space+/*skip alloc id*/1) : 0;
    45   return space_base(x, space_index(x), base);
    46 }
    47
    48 int space_base(const reagent& x, int space_index, int base) {
    49   if (space_index == 0)
    50     return base;
    51   double chained_space_address = base+/*skip length*/1+/*skip alloc id of chaining slot*/1;
    52   double chained_space_base = get_or_insert(Memory, chained_space_address) + /*skip alloc id of chained space*/1;
    53   return space_base(x, space_index-1, chained_space_base);
    54 }
    55
    56 int space_index(const reagent& x) {
    57   for (int i = 0;  i < SIZE(x.properties);  ++i) {
    58     if (x.properties.at(i).first == "space") {
    59       if (!x.properties.at(i).second || x.properties.at(i).second->right)
    60         raise << maybe(current_recipe_name()) << "/space metadata should take exactly one value in '" << x.original_string << "'\n" << end();
    61       return to_integer(x.properties.at(i).second->value);
    62     }
    63   }
    64   return 0;
    65 }
    66
    67 :(scenario permit_space_as_variable_name)
    68 def main [
    69   space:num <- copy 0
    70 ]