Switch Expressions

The switch-expression in Minilang provides an alternative to an if-expression with multiple elseif branches when a single value is tested for the first match.

For example:

fun test(X) do
   if X = 1 then
      "one"
   elseif 2 <= X <= 3 then
      "few"
   elseif 4 <= X <= 7 then
      "several"
   else
      "many"
   end
end

fun test2(X) do
   switch X: integer
   case 1 do
      "one"
   case 2, 3 do
      "few"
   case 4 .. 7 do
      "several"
   else
      "many"
   end
end

There are a few differences, a switch-expression needs a switch provider, in this case integer. The provider of a switch determines how Minilang treats the switch values to find a matching branch. A switch-expression also allows multiple values per-branch, and overall is faster than the corresponding if-expression. Note that the values in each branch are evaluated at compile time so can't refer to local variables.

Given a switch-expression with provider P, the compiler uses the method compiler::switch(P, Cases...) to implement the switch-expression. Here Cases is a list of lists, one list per case containing the case values. The implementation of compiler::switch for P must return another function, which takes a value and return an integer index (starting at 0) corresponding to the chosen branch.

This design allows support for new kinds of switch-expressions to be added later, either to the language or by specific applications. Currently the following switch providers are available:

type

Case values must be types (or nil). A case value matches if the switch value is of the same type or a sub-type.

integer, real

Case values must be numbers or numeric ranges. A case value matches if the switch value is equal to it (for numbers) or contained in it (for ranges).

string

Case values must be strings or regular expressions. A case value matches if the switch value is equal to it (for strings) or matches it (for regular expressions).

Any enum type

Case values must be values of the enum type or strings corresponding to values of the enum. A case value matches if the switch value is equal to it.

Any flags type

Flag values must be values of the flags type, strings corresponding to values of the enum, or tuples of the previous. A case value matches if the switch value contains at least the same flags, it may have extra flags.

Any function or macro

Any function or macro can be used as a switch provider as long as it accepts a list of lists and returns another function as described above.