Comparing Erlang and Prolog
In the back of my mind, I have for a little while known two things:
- Prolog is no language for a man to use.
- I should probably check out this Erlang thing.
So today I have a look at this getting-started-with-erlang tutorial, and lo! Perhaps my Prolog skills are not so useless after all.
% Erlang
-module(tut4).
-export([list_length/1]).
% simple function to return list length
list_length([]) ->
0;
list_length([_F|R]) ->
1 + list_length(R).
% Prolog
:- module(tut4).
:- export list_length/2.
% simple predicate to return list length
list_length([], 0).
list_length([_F|R], L):-
L is 1 + list_length(R).
Similarities so far:
- Modules - one per file.
- Module/export directives very similar.
- Commas go between statements, full stops end code blocks.
- Comments use the same symbol.
- Multiple function clauses with argument matching.
- Lists work in same manner: [head|tail].
- Variables begin with capitals.
- Atoms begin with lowercase, (atoms are the same as Ruby’s symbols).
- Singleton variables generate compiler warnings, unless they are prefixed with an underscore (_F above)
- Both scope modules with a colon.
- To quit the shell you call ‘halt’.
Differences:
- Erlang functions have return values, rather than the Prolog concept of success and failure, and passing return values out through arguments.
About
Daniel Lucraft, London, UK. Interested in Ruby, Prolog and software.