CODEWITHSUNDEEP
variablesprolognaming
Wouter Beek
Wouter Beek

Reputation: 3407

How to consistently replace atoms with variables in Prolog?

I am writing a parser for SPARQL (Semantic Web query language) using DCG. I want to replace SPARQL variable names with Prolog variables. How would I go about this?

I can generate new variables using length([NewVar], 1), but I cannot keep track of existing assignments by simply using a list of name-variable pairs. A member/2 operation on the list will return a new variable, not the one stored in the list.

Is there an easy way for naming variables in Prolog, e.g., '$VAR(Name)'?

Upvotes: 6

Views: 1055

Answers (1)

user502187
user502187

Reputation:

member/2 will do what you want. Here is an example:

Welcome to SWI-Prolog (Multi-threaded, 64 bits, Version 7.3.25)
Copyright (c) 1990-2016 University of Amsterdam, VU Amsterdam

L=[a-X,b-Y,c-Z], member(b-V,L).
L = [a-X, b-V, c-Z],
Y = V

But you might get problems if you interleave write/1 with member/2, since a variable might change its identity, i.e. the write symbol in the following circumstances:

  • because of garbage collection, if a variable is written as _G<memloc>
  • because of aliasing, in the above example the memloc of V might be shown
    instead of the memloc of Y

Same problem with (@<)/2. One way out is to use attribute variables, which at least puts an end to aliasing, since attribute variables are usually unified last,

so in the above example if Y is an attribute variable and V is an ordinary variable you would never see the memloc of V after calling member/2.

Further you can also mitigate the problem by using ISO core standard variable_names/1 write option, to write out a variablified term. The variable_names/1 write option is immune to garbage collection or aliasing.

Bye

Upvotes: 2

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