CODEWITHSUNDEEP
prologswi-prolog
921kiyo
921kiyo

Reputation: 572

How to make list constructors (e.g .(a,[]) == [a].) work in Prolog?

I am learning Prolog and .(a,[]) == [a]. should return true in SWI-Prolog, but it gives me an error saying 

ERROR: Type error: `dict' expected, found `a' (an atom)
ERROR: In:
ERROR:   [11] throw(error(type_error(dict,a),_4020))
ERROR:   [10] '$type_error'(dict,a) at /Applications/SWI-
Prolog.app/Contents/swipl/boot/init.pl:3369
ERROR:    [9] '$dicts':'.'(a,[],_4086) at /Applications/SWI-
Prolog.app/Contents/swipl/boot/dicts.pl:46
ERROR:    [8] '<meta-call>'(user:(...,...)) <foreign>
ERROR:    [7] <user>

Anyone knows how to fix this?

Upvotes: 2

Views: 105

Answers (1)

Will Ness
Will Ness

Reputation: 71119

Start the SWI Prolog executable with the --traditional command line option (comment due to user:false).

Then it works:

1 ?- .(a,[]) == [a].
true.

2 ?- current_prolog_flag( traditional, X).
X = true.

3 ?- set_prolog_flag( traditional, false).
ERROR: set_prolog_flag/2: No permission to modify flag `traditional'

4 ?-

This is mentioned in the documentation here (see "traditional", near the bottom of the page).

As can be seen, attempting to change it from within the running SWI session, fails.

Upvotes: 2

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