The + procedure left-folds its arguments in interpreted code and right-folds its arguments in compiled code. This may or may not be a bug.
Obviously, with exact numbers the direction of folding makes no difference. But the difference is easily seen with flonums, as flonum addition is necessarily non-associative. For example, where flonums are IEEE doubles: scheme@(guile-user)> ,o interp #f scheme@(guile-user)> (+ 1.0 (expt 2.0 -53) (expt 2.0 -53)) $1 = 1.0000000000000002 scheme@(guile-user)> (+ (expt 2.0 -53) (expt 2.0 -53) 1.0) $2 = 1.0 scheme@(guile-user)> ,o interp #t scheme@(guile-user)> (+ 1.0 (expt 2.0 -53) (expt 2.0 -53)) $3 = 1.0 scheme@(guile-user)> (+ (expt 2.0 -53) (expt 2.0 -53) 1.0) $4 = 1.0000000000000002 Compiler and interpreter agree when the order of operations is explicitly specified: scheme@(guile-user)> (+ (+ 1.0 (expt 2.0 -53)) (expt 2.0 -53)) $5 = 1.0 scheme@(guile-user)> (+ 1.0 (+ (expt 2.0 -53) (expt 2.0 -53))) $6 = 1.0000000000000002 If your flonums are not IEEE double then the exponent in the test case has to be adapted. R5RS and the Guile documentation are both silent about the order of operations in cases like this. I do not regard either left-folding or right-folding per se as a bug. A portable Scheme program obviously can't rely on a particular behaviour. My concern here is that the compiler and interpreter don't match, making program behaviour inconsistent on what is notionally a single implementation. That mismatch may be a bug. I'm not aware of any statement either way on whether you regard such mismatches as bugs. (An explicit statement in the documentation would be most welcome.) R6RS does have some guidance about the proper behaviour here. The description of the generic arithmetic operators doesn't go into such detail, just describing it as generic. It can be read as implying that the behaviour on flonums should match the behaviour of the flonum-specific fl+. The description of fl+ (libraries section 11.3 "Flonums") says it "should return the flonum that best approximates the mathematical sum". That suggests that it shouldn't use a fixed sequence of dyadic additions operations, and in my test case should return 1.0000000000000002 regardless of the order of operands. Obviously that's more difficult to achieve than just folding the argument list with dyadic addition. Interestingly, fl+'s actual behaviour differs both from + and from the R6RS ideal. It left-folds in both compiled and interpreted code: scheme@(guile-user)> (import (rnrs arithmetic flonums (6))) scheme@(guile-user)> ,o interp #f scheme@(guile-user)> (fl+ 1.0 (expt 2.0 -53) (expt 2.0 -53)) $7 = 1.0 scheme@(guile-user)> (fl+ (expt 2.0 -53) (expt 2.0 -53) 1.0) $8 = 1.0000000000000002 scheme@(guile-user)> ,o interp #t scheme@(guile-user)> (fl+ 1.0 (expt 2.0 -53) (expt 2.0 -53)) $9 = 1.0 scheme@(guile-user)> (fl+ (expt 2.0 -53) (expt 2.0 -53) 1.0) $10 = 1.0000000000000002 fl+'s behaviour is not a bug. The R6RS ideal is clearly not mandatory, and the Guile documentation makes no stronger claim than that its fl+ conforms to R6RS. As it is consistent between compiler and interpreter, it is not subject to the concern that I'm raising in this ticket about the generic +. -zefram