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Asked: 2026-05-30T00:16:10+00:00

I am recently learning about Prolog and I find the three types used for

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I am recently learning about Prolog and I find the three types used for defining infix operators confusing.

What are the differences between xfx, xfy and yfx when specifying the type of an operator?
I have googled about the problem and haven’t found anything useful.

I tried typing the following codes in Prolog:

:- op(500,yfx,is_alive).
is_alive(A,B) :- display([A,B]).
:- op(500,xfy,is_alive2).
is_alive2(A,B) :- display([A,B]).
:- op(500,xfx,is_alive3).
is_alive3(A,B) :- display([A,B]).

and the output:

| ?- 1 is_alive 2.
'.'(1,'.'(2,[]))

yes
| ?- 1 is_alive2 2.
'.'(1,'.'(2,[]))

yes
| ?- 1 is_alive3 2.
'.'(1,'.'(2,[]))

yes

The results showed no difference to me.

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1 Answer

  1. Editorial Team

    The ‘type’ of the operator controls associativity, e.g. whether expression “5 – 4 – 3” should be interpreted as “(5 – 4) – 3”, which is known as left associativity or as “5 – (4 – 3)”, which is right associativity.

    If the minus operator should work as usually expected, is needs to be defined as ‘yfx’ which makes it left-associative. Other types as ‘xfx and ‘xfy’ are variations on this. See more at http://www.cse.unsw.edu.au/~billw/cs9414/notes/prolog/op.html.

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