There's a very good StackOverflow question which covers this: "When is memoization automatic in GHC?"[1]. I found it really cleared up the issue for me.

[1]: http://stackoverflow.com/questions/3951012/when-is-memoization-automatic-in-ghc-haskell

On Sat, Jun 15, 2013 at 9:13 PM, Clark Gaebel <cgaebel@uwaterloo.ca> wrote:

Yes. In general, GHC won't CSE for you.

- Clark

On Saturday, June 15, 2013, Christopher Howard wrote:
On 06/15/2013 04:39 PM, Tommy Thorn wrote:
>
>
> There's not enough context to answer the specific question,
> but lazy evaluation isn't magic and the answer is probably "no".
>
> Tommy
>

Perhaps to simplify the question somewhat with a simpler example.
Suppose you have

code:
--------
let f x = if (x > 4) then f 0 else (sin x + 2 * cos x) : f (x + 1)
--------

After calculating at x={0,1,2,3}, and the cycle repeats, are sin, cos,
etc. calculated anymore?

--
frigidcode.com

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