Philip Scott wrote:
Daniel Fischer wrote:
Am Freitag 16 April 2010 16:25:08 schrieb Heinrich Apfelmus:
Philip Scott wrote:
What I want is something like putStrLn that doesn't print anything, and takes any type as its input, evaluates it to normal form and then does nothing else. It might be called something like 'evaluate' have a type like this
evaluate :: a -> IO()
With Hoogle, you can search for types:
http://haskell.org/hoogle/?hoogle=a+-%3E+IO+%28%29
12th result from the top.
That evaluates only to WHNF.
Control.DeepSeq.rnf seems to be closer, but requires NFData instances.
The documentation says that, but it does appear to go deeper than just one level:
Debug.Trace Prelude Control.Exception> let a = trace "Hello" 42 Debug.Trace Prelude Control.Exception> let b = trace "Frank" (a * 2) Debug.Trace Prelude Control.Exception> evaluate b Frank Hello 84
Perhaps it specifies WHNF in case you 'evaluate' something which doesn't have a HNF (like a partially applied function?).
"evaluate x" is defined as "return $! x". This indeed evaluates the value immediately then and there. It *only* evaluates to WHNF (weak head normal form). For number values, WHNF (= evaluate to outer constructor only) and NF (= completely evaluate) are the same thing, as shown in your example above. The (a * 2) makes no difference, because whether WHNF = NF or not depends on the type of the value only, not the expression used to calculate it. To evaluate x to NF (completely) then and there in general, you have to say evaluate (rnf x) where rnf is defined in Control.DeepSeq in the 'deepseq' package. The type of x has to be an NFData instance. Haskell does not provide any other way to force evaluation to NF other than to explicitly use `seq` on each element of the structure. The purpose of DeepSeq is to make this task easier. Steve