Is suspect this being a bug: When a recursive function call is used as a
default value in a map, its tail position is not recognized. The problem
can be easily demonstrated using the fixpoint function. The fixpoint
function is usually defined as
> (defn fix [f x] (let [v (f x)] (if (= v x) x (recur f v))))
To avoid the 'if' special form, we can use a map. The compiler rejects
compilation and does not recognize (recur f v) to be correctly in tail
position.
> (defn fix2 [f x] (let [v (f x)] ({x x} v (recur f v))))
CompilerException java.lang.UnsupportedOperationException: Can only recur
from t
ail position, compiling:(NO_SOURCE_PATH:41)
Avoiding tail recursion lets fix2 compile but not execute properly:
> (defn fix2 [f x] (let [v (f x)] ({x x} v (fix2 f v))))
#'user/fix2
> (fix identity 1)
1
> (fix2 identity 1)
StackOverflowError user/fix2 (NO_SOURCE_FILE:44)
Strange, isn't it!?
Dominikus
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