[m-users.] Error instance Mercury

Maxime Gautin maxime.gautin at gmail.com
Tue Jun 26 22:23:19 AEST 2018

Hello everyone,

I have a compilation fail with the code I am studying. It is about an

:- type token_list == list(token).

:- instance parser_state(token_list) where [

        get_token(T,[T | Ts], Ts),

        unget_token(T, Ts) = [T | Ts]

token is a type defined on another file.

The typeclass is also declared on another file :

:- typeclass parser_state(T) where [
    pred get_token(token, T, T),
    mode get_token(out, in, out) is det,
    func unget_token(token, T) = T,
    mode unget_token(in, in) = out is det

And this is the error :

imputgen_bin.m:061: In instance declaration for
imputgen_bin.m:061:   `int_parser.parser_state(list.list(scanner.token))':
imputgen_bin.m:061:   the first argument is a type whose first argument is
imputgen_bin.m:061:   a variable.
imputgen_bin.m:061:   (Types in instance declarations must be functors with
imputgen_bin.m:061:   variables as arguments.)

I am not very familiar with instance in Mercury, and I don't understand
what I have to modify. I hope you can help me.


Maxime Gautin
Polytech Marseille
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