HOL_ERR

`HOL_ERR`

   HOL_ERR : hol_error -> exn

Standard HOL exception.

HOL_ERR is the exception that HOL functions are expected to raise when they encounter an anomalous situation.

Example

A HOL_ERR value is built from a hol_error value, which is typically created with mk_hol_error.

   > val test_exn =
       HOL_ERR (mk_hol_error "Foo" "bar"
                    locn.Loc_Unknown "unexpected input")

   > raise test_exn;
   Exception- HOL_ERR (at Foo.Bar: unexpected input) raised

HOL_ERR values can also be directly constructed by mk_HOL_ERR or mk_HOL_ERRloc.

   > val test_exn_again =
       mk_HOL_ERR "Foo" "bar" "unexpected input"

   > raise test_exn_again;
   Exception- HOL_ERR (at Foo.Bar: unexpected input) raised

Usage patterns

Constructing backtraces

Information can be added to a HOL_ERR with wrap_exn:

   > raise wrap_exn "structA" "fnB" test_exn;
   Exception-
      HOL_ERR
        (at structA.fnB:
           at Foo.bar: unexpected input) raised

Location information can be included with wrap_exn_loc.

A common HOL programming idiom using wrap_exn has the following pattern (assume function bar is being defined in structure Foo):

   fun bar x y =
     let val z = ...
     in
        ...
     end
     handle e as HOL_ERR _ => raise wrap_exn "Foo" "bar" e

If HOL_ERR <holerr> happens to be raised inside an invocation of bar, the handler will extend the origins of holerr with Foo and bar and raise the augmented HOL_ERR.

Scrutinizing and setting the payload

The payload of a HOL_ERR can be accessed by pattern matching and the contents accessed by functions over hol_error such as top_structure_of, top_function_of, and message_of:

   > val HOL_ERR holerr = test_exn

   > val (s,f,m) = (top_structure_of holerr,
                    top_function_of holerr,
                    message_of holerr)
   val f = "bar": string
   val m = "unexpected input": string
   val s = "Foo": string

Portions of the payload can also be set by set_top_function and set_message.

Branching on the interaction mode

The variable Globals.interactive is used by programs to tell whether the HOL4 system is running interactively (i.e. is in the Read-Eval-Print loop) or not (is running in batch mode under Holmake). In the REPL, an uncaught HOL_ERR propagates to the top level and gets prettyprinted. In batch mode, in contrast, uncaught exceptions are not prettyprinted and can be rendered quite messily. The function render_exn can be used to write code that displays HOL_ERR properly in either mode.

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