[Haskell-cafe] evaluation semantics of bind

Tillmann Rendel rendel at cs.au.dk
Mon Feb 9 12:06:44 EST 2009

Gregg Reynolds wrote::
> My original question was motivated by the observation that a human reader of
> an expression of the form "e >>= f" , on seeing that f is constant, may pull
> the constant value out of f, disregard e and dispense with the application f
> e.  

While a human reader may well do that, but it would be correct or wrong 
depending on the definition of >>=. The same is of course true for 
compilers. By the way, there is no "application f e".

An example where it would be wrong to ignore e:

   sum ([1, 2] >>= const [21])

This expression should evaluate to sum [21, 21] = 42, not sum [21] = 21.

There is nothing special with IO or >>=, so there is no need to 
introduce special cases for IO or >>= in a formal or informal semantics 
of Haskell.


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