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+# Only Booleans have truthiness
+
+Like in Java, there should be no implicit conversion of values to a
+Boolean.  This leads to sloppy code and subtle bugs.
+
+I believe the code base of Guix contained an example of this at some
+point: Build phases ending in a call to `system*` would return 0 or 1
+which would pass as "true" regardless.
+
+In most cases, you should know the actual type of the value you're
+receiving, and do an appropriate check, be it `zero?`, `null?`, or
+some other check.  If you truly want to check if something is "any
+value other than false" then you can always do:
+
+```scheme
+
+(if (not (eq? #f value))
+    (do something))
+
+```
+
+No, you cannot use `(not (not x))` because `not` obviously expects a
+Boolean argument!  Duh.
+
+I'm actually serious about this.  Scheme went all the way to make null
+separate from false, but then refused to go all the way and decided to
+allow non-Boolean values to function as Booleans anyway.
+
+Of course, performing a type-check on every single conditional may
+incur a serious performance penalty.  If so, then the same flag that
+determines [whether returned values can be ignored](strict-mode.html)
+may also determine whether non-Booleans can be coerced into Booleans.