<html> <head> <meta http-equiv="content-type" content="text/html; charset=UTF-8"> </head> <body> Hi everyone, I confess to being a bit befuddled at the somewhat terse and elusive definition given by the reference manual on Type types (3.1.2.2): <blockquote> "The tuple types are {}, {T}, {T1, T2}, . . . A tuple type is equivalent to a discriminated union type (see Section 3.2.1 [Discriminated unions], page 33) with declaration :- type {Arg1, Arg2, ..., ArgN} ---> { {Arg1, Arg2, ..., ArgN} }." </blockquote> The equivalence with discriminated union types is clear; the type declaration is a bit less so. My understanding of tuples would rather be as follows: <blockquote> :- type {Arg1, Arg2, ..., ArgN} ---> {} (Arg1, Arg2, ..., ArgN). </blockquote> In other words, tuple terms look to me as (instantiated) discriminated union types with anonymous constructors '{}' of arity N consisting of the two joined tokens '{' and '}'. This is evidenced among others by tests/hard_coded/tuple_test.m:30. I think the reference manual could perhaps rephrase its explanation in a more convincing and clear-cut way as above. I honestly do not understand the manual definition; to me it irresistibly smacks of infinite recursion. Something vaguely Chomskian in me, against all odds, keeps rewriting this declaration as follows: :- type {Arg1, Arg2, ..., ArgN} ---> { { { Arg1, Arg2, ..., ArgN} } }. :- type {Arg1, Arg2, ..., ArgN} ---> { { { { Arg1, Arg2, ..., ArgN} } } }. :- type {Arg1, Arg2, ..., ArgN} ---> { { { { { Arg1, Arg2, ..., ArgN} } } } }. (... to Infinity ...) somewhat hellishly. But there must be something I missed there; I would be grateful for any principled denial of these misconceptions of mine (while we are all still alive). Fabrice </body> </html>