All persistent data structures are immutable, but not all immutable data structures are persistent.
For example, imagine an immutable array that, unlike Clojure's vector data structure, implemented "conj" by copying the entire array into a new one with the original elements plus the new one. Immutable, but the performance characteristics are abysmal. As a more subtle example, imagine an immutable array that, unlike Clojure's vector data structure, let you "update" arbitrary elements, and did not copy the entire array, but created an imbalanced tree of vector indices that had their values "updated". A lookup in this augmented data structure would require first looking through the imbalanced tree for the index, to see if it had been modified, and only if it failed to find it in the tree would it be looked up in the original array. Immutable, but not persistent in the sense that it maintains the performance characteristics of the original data structure -- the augmented/updated data structure has slow access times than the original. Andy On Sun, Sep 25, 2011 at 11:34 AM, Dennis Haupt <d.haup...@googlemail.com>wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > so there is no difference. > > Am 25.09.2011 15:28, schrieb Stuart Halloway: > >> what's the difference between persistent and immutable? > > > > See http://en.wikipedia.org/wiki/Persistent_data_structure, which > > now has a nice shout out to Clojure. > > > > Stu > > > > > - -- > > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v2.0.14 (MingW32) > Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ > > iQIcBAEBAgAGBQJOf3QyAAoJENRtux+h35aGsKQP/jHBNqhUCQfSQ7XjCe3vTlF5 > zhjcXmyhWbRUF7fEsrmljtpOu630gAnl51cmlBOwpOQXLmGJg386422GzUtRexG3 > A4KcrYEahAKBK5R1Tiu3WMqAr/h3t9oYi6APYaU29qJqqO6lnZR/bp1yAR+wnmZT > ausVGzUlE+p5DlfWHAMaAsEVYp1XX282BKecgr/cHsBy9Jwl2NdWQl0Ss/ZYPj7x > JE04/y6T/1jomWdM+dwXZ/oCucWmDjSgg3nHMUy/P9yab0kN2qHprqomBCBQeOpj > zY5KS2/0x6nS9XpNfKF1f7VdqG8RkVdE+iE14a5uzCWzAQfMbeZUQcyOv8H8QQXU > v0VYO1htvWwoJRqoCUan5UhrucM+LDqFEml7n8S4y7kbWWy9CJHD0bq9VjgIBIJY > oo8VrFM7ciO+9mmJ7VfdNmUKPmcclWVFp3PVbNKJVMFXO7s+Myj5y/irWDJ3fBTY > VYiCKNtbU9uaNZRVfyTQhVF5i+607BfH78S6wA1fEXYWWcvNiwEBoTaKi4cHh6wF > nbaVf2xOLahJngEg6m8yBQfcnRm5H6WZ2h53UA+c1AgwiRgOAt+FGiK6J695EZbp > LlslJGUH9QVZXsDVWPthtr2Qk8fJtT7c7UQksfYU+F8lANtyTMcN/kAQ2ngZ2pbq > aLOthkidIm51E9NkBRH5 > =SNNL > -----END PGP SIGNATURE----- > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with > your first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
