Bliki: Paracelsus Maxim

The difference between a medicine and a poison is dosage. Often we talk about certain habits, in programming or life, are good or bad. But few things are simple binaries. Some vary with context: reading a book is a good thing sitting in my garden, but not while driving my car. But another variable is dosage: a little pain-killer salves my headache, but too much will kill me. The importance of dosage was noticed by a 16th century Swiss physician called Paracelsus. His quote was originally in German “Alle Dinge sind Gift, und nichts ist ohne Gift; allein die Dosis macht, dass ein Ding kein Gift ist.” which (according to Wikipedia) translates as “All things are poison, and nothing is without poison; the dosage alone makes it so a thing is not a poison.” It's also known as “The dose makes the poison” or if you prefer your sayings in Latin “dosis sola facit venenum”. In programming, global data is a good example of the Paracelsus Maxim (as I like to call it). A little global data, especially when immutable, can be a handy way of propagating information that may needed anywhere in a program, but it quickly becomes dangerous if there is a lot of it about. This kind of thing crops up in l…

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