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To be warned of arrays being unpacked, you can do SetSystemOptions[PackedArrayOptions->UnpackMessage->True] or, in versions after 7, On["Packing"] (thanks to OleksandrR for pointing this out). The you see that eg Select unpacks: try Select[fpacked, 3] and a message is produced. Also assigning a value of different type to a packed array unpacks it: try fpacked[[2]] = 4 to see this.

So, you could set $Post = Developer`PackedArrayForm and then packed arrays would be displayed in a special way. I am not sure if this has any other sideeffects (this has been suggested in this great answer by ruebenko).

To be warned of arrays being unpacked, you can do SetSystemOptions[PackedArrayOptions->UnpackMessage->True] or, in versions after 7, On["Packing"] (thanks to OleksandrR for pointing this out). The you see that eg Select unpacks: try Select[fpacked, 3] and a message is produced. Also assigning a value of different type to a packed array unpacks it: try fpacked[[2]] = 4 to see this.

So, you could set $Post = Developer`PackedArrayForm and then packed arrays would be displayed in a special way. I am not sure if this has any other sideeffects (this has been suggested in this great answer by ruebenko).

To be warned of arrays being unpacked, you can do SetSystemOptions[PackedArrayOptions->UnpackMessage->True]. The you see that eg Select unpacks: try Select[fpacked, 3] and a message is produced. Also assigning a value of different type to a packed array unpacks it: try fpacked[[2]] = 4 to see this.

To be warned of arrays being unpacked, you can do SetSystemOptions[PackedArrayOptions->UnpackMessage->True] or, in versions after 7, On["Packing"] (thanks to OleksandrR for pointing this out). The you see that eg Select unpacks: try Select[fpacked, 3] and a message is produced. Also assigning a value of different type to a packed array unpacks it: try fpacked[[2]] = 4 to see this.

f = Developer`FromPackedArray[RandomReal[{-1, 1}, 50000]];
fpacked = Developer`ToPackedArray[RandomReal[{-1, 1}, 50000]];
timeIt[f.f]
timeIt[fpacked]
(*
0.000563103
4.57183\[Times]10^-8
*)

ClearAll[f, fpacked];
f = Developer`FromPackedArray[RandomReal[{-1, 1}, 500000]];
fpacked = Developer`ToPackedArray[RandomReal[{-1, 1}, 500000]];

fpacked.fpacked // timeIt
f.f // timeIt

Sin[fpacked] // timeIt
Sin[f] // timeIt

(*
0.0001610173
0.01167263
0.00487482
0.01420070
*)