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prologappend

Prolog, understanding append/3

?- append([], [X1], [a,b]).

Why does this return no and not 

X1 = a,b

Since 

? - append([], [a,b], [a,b])

returns yes?

Solution

  • See: append/3

    append(?List1, ?List2, ?List1AndList2)

    List1AndList2 is the concatenation of List1 and List2

    So for

    ?- append([], [X1], [a,b]).

    [] is the empty list and [X1] is a list with a variable X1

    If you run the query like this

    ?- append([],[X1],A).

    you get

    A = [X1].

    which means that A is the concatenation of [] and [X1].

    In your query it is asking if the concatenation of [] and [X1] is [a,b] which is false, or no.

    For your second query

     ? - append([], [a,b], [a,b])

    it is asking if the concatenation of [] and [a,b] is [a,b] which is true, or yes.