[Haskell-cafe] Help with another Bird problem (3.3.4)

[Luke Palmer]

On Tue, Feb 24, 2009 at 9:53 AM, Peter Hilal <peter at hilalcapital.com> wrote:

> Could someone provide a solution to do the following problem from Bird:
>
> The function "log" can be specified by the condition that log (2^m) = m for
> all m.  Construct a program for log and prove that it meets the
> specification.


Well, here is the least solution on Nat:

log n = head [ m | m <- [0..], 2^m == n ]

This meets the specification, and is _|_ everywhere else :-)  The proof is
trivial.

Luke
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