diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 74 |
1 files changed, 73 insertions, 1 deletions
@@ -16847,7 +16847,7 @@ then the list is terminated, and no more items are produced. .desc The .code expand-right -is a complement to +function is a complement to .codn reduce-right , with lazy semantics. @@ -16912,6 +16912,78 @@ then the lazy list ends. --> (5 4 3 2 1 0) .cble +.coNP Functions @ expand-left and @ nexpand-left +.synb +.mets (expand-left < gen-fun << value ) +.mets (nexpand-left < gen-fun << value ) +.syne +.desc +The +.code expand-left +function is a companion to +.codn expand-right . + +Unlike +.codn expand-right , +it has eager semantics: it calls +.code gen-fun +repeatedly and accumulates an output list, not returning +until +.code gen-fun +returns +.codn nil . + +The semantics is as follows. +.code expand-left +initializes an empty accumulation list. Then +.meta gen-fun +is called, with +.meta value +as its argument. + +If +.meta gen-fun +it returns a cons cell, then the +.code car +of that cons cell is pushed onto the accumulation list, +and the procedure is repeated: +.meta gen-fun +is called again, with +.code cdr +taking the place of +.meta value. + +If +.meta gen-fun +returns +.codn nil , +then the accumulation list is returned. + +If the expression +.code (expand-right f v) +produces a terminating list, then the following equivalence holds: + +.cblk + (expand-left f v) <--> (reverse (expand-right f v)) +.cble + +Of course, the equivalence cannot hold for arguments to +.code expand-left +which produce an infinite list. + +The +.code nexpand-left +function is a destructive version of +.codn expand-left . + +The list returned by +.code nexpand-left +is composed of the cons cells returned by +.code gen-fun +whereas the list returned by +.code expand-left +is composed of freshly allocated cons cells. + .coNP Function @ repeat .synb .mets (repeat < list <> [ count ]) |