[m-users.] Error: non-abstract instance declaration in module interface.

Sean Charles (emacstheviking) objitsu at gmail.com
Thu Feb 3 08:39:26 AEDT 2022

I am trying t ouse typeclasses for the first time ever and, well, it's not been the happy time I wanted !

I have a bunch of types for the arguments for the various rendering instructions and I wanted to be able to have a uniform means to generate error messages and other actions across that set of types, for example:

:- type leaf
    --->    tk(location, string)
    ;       kw(location, string)
    ;       s1(location, string)
    ;       s2(location, string).

:- type arglist_spec
    --->    arg_leaf(leaf)
    ;       arg_def(ps, render_inst)
    ;       arg_ref(ps).

:- type gencall_spec
    --->    gc_leaf(leaf)
    ;       gc_arg(render_inst)
    ;       gc_keyword(ps, render_inst) % python-like, foo=bar
    ;       gc_ref(ps).                 % C or PHP: &foo

:- type call_term
    --->    ct_leaf(leaf)
    ;       ct_inst(render_inst).

The plan was to be able to call something like `make_error` with one of the above, by supplying the error code and the instance that was being rejected, something like:

    Error = make_error(TheInstance, incorrect_form_error)

and so I have tried many things, all of them wrong, but I seme to have exhausted the number of wrong ways of doing it without actually finding the correct way to do it. I am still not sure my use case is even what typeclasses are for but it feels correct, from the reference manual section 10.1,

A type class is a name for a set of types (or a set of sequences of types) for which cer- tain predicates and/or functions, called the methods of that type class, are defined. A ‘typeclass’ declaration defines a new type class, and specifies the set of predicates and/or functions that must be defined on a type (or sequence of types) for it (them) to be considered to be an instance of that type class.

so if I have understood it, my set of types are the ones listed above, and the methods (one so far) are make_error so far. I also want to have a function to return a string representation of each instance as well.

Here is my awful attempt,  syntax_error, error_msg are types defined elsewhere as is the function nowhere,

:- typeclass tstring(T) where [
    func make_error(T::in, error_msg::in) = (syntax_error::out) is det

:- instance tstring(leaf) where [   LINE 73 <===
    make_error(_X, E) = syntax_error(nowhere, error, E)

...which gives this error:

datatypes.m:073: Error: non-abstract instance declaration in module interface.
Making Mercury/ints/datatypes.int
datatypes.m:073: Error: non-abstract instance declaration in module interface.
** Error making `Mercury/ints/datatypes.int'.
make[2]: *** [felt] Error 1


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