MonadFix instance required

Hi, I need a MonadFix instance for the Put monad so I can use the length of an expressions output as a parameter to construct the expression (for example, to output the offset to the end of a header at some place within the header itself).

I attempted to write the instance of MonadFix but there is excess strictness somewhere that causes the fixedpoint to be undefined: https://github.com/code5hot/binary/commit/551591dd05e3cb7769a2d61fdc16d8f45a69b88e#commitcomment-34039617

Any suggestions on how to complete this mfix implementation?

Start with the attempted MonadFix implementation in commit 551591dd05e3cb7769a2d61fdc16d8f45a69b88e and inspect the Put monad's strictness around the fixed point. Done means a MonadFix instance can use an expression's output length while constructing that expression without producing an undefined fixed point.

Written by the indexing model from the issue text.