4227 - second example implementing exceptions

I think I like this better. It doesn't violate the type system. Still two
sources of klunkiness:

a) We need to add dead code because we don't consider exceptions when we
add implicit 'return' statements to functions. Should be easy to fix.
b) Still no way to create a generic 'try' function. This will be hard to
fix.

1 files changed, 64 insertions, 0 deletions

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+# Example program showing exceptions built out of delimited continuations.
+# Slightly less klunky than exception.mu.
+
+# Since Mu is statically typed, we can't build an all-purpose higher-order
+# function called 'try'; it wouldn't know how many arguments the function
+# passed to it needs to take, what their types are, etc. Instead, until Mu
+# gets macros we'll directly use the continuation primitives.
+
+exclusive-container error-or:_elem [
+  error:text
+  value:_elem
+]
+
+def main [
+  local-scope
+  no-exception:bool <- copy 0/false
+  foo 0/no-exception
+  raise-exception:bool <- copy 1/true
+  foo 1/raise-exception
+]
+
+# example showing exception handling
+def foo raise-exception?:bool [
+  local-scope
+  load-inputs
+  # To run an instruction of the form:
+  #   try f ...
+  # write this:
+  #   call-with-continuation-mark 999/exception-tag, f, ...
+  # By convention we reserve tag 999 for exceptions.
+  #
+  # The other inputs and outputs to 'call-with-continuation-mark' depend on
+  # the function it is called with.
+  _, result:error-or:num <- call-with-continuation-mark 999/exception-tag, f, raise-exception?
+  {
+    val:num, normal-exit?:bool <- maybe-convert result, value:variant
+    break-unless normal-exit?
+    $print [normal exit; result ] val 10/newline
+  }
+  {
+    err:text, error-exit?:bool <- maybe-convert result, error:variant
+    break-unless error-exit?
+    $print [error caught: ] err 10/newline
+  }
+]
+
+# Callee function that we catch exceptions in must always return using a
+# continuation.
+def f raise-exception?:bool -> result:error-or:num [
+  local-scope
+  load-inputs
+  {
+    break-unless raise-exception?
+    # throw/raise
+    result <- merge 0/error, [error will robinson!]
+    return-continuation-until-mark 999/exception-tag, result
+  }
+  # 'normal' return; still uses the continuation mark
+  result <- merge 1/value, 34
+  return-continuation-until-mark 999/exception-tag, result
+  # dead code just to avoid errors
+  result <- merge 1/value, 0
+  return result
+]