[Haskell-beginners] some insights into functional programming
adam at edea.se
Sun Aug 9 21:07:44 EDT 2009
The reason my is available in the lambda \x -> my >>= \y -> W (x+y)
has to do with scoping rules, Haskell (and almost all programming
languages) use static scoping, meaning a variable defined in an outer
function is available in the inner function, for instance, (\a -> \b -
> (a,b)) 1 2 will evauate to (1,2) since a is bound to 1 in the outer
lambda, and the inner one can refer to the outer one, if instead you
write (\a -> \a -> (a,a)) 1 2 the result will be (2,2) since the
innermost a will be used (no ambiguity here, but if shadowing is done
by accident it can be hard to find the error). The book Structure and
Interpretation of Computer Programs (freely available online)
discusses this subject in detail.
>>= is available inside the lambda for the same reason, >>= is
imported into the modules namespace, and therefore available
everewhere, unless a shadowing binding exists.
I wouldn't say that this has anything to do with functional
This also exists in Python i might add, you can read variables defined
in an outer scope:
>>> a = 1
>>> def f():
... return a
The fact that most languages are scoped like this is nothing obvious,
and like I said, Python is the same.
I hope this helps!
On Aug 9, 2009, at 9:31 PM, Michael Mossey wrote:
> I'm starting to figure out a few things that I didn't "get" about
> functional programming and monads. I wanted to explain them. I'm not
> looking for a particular response to this post, other than any
> elaboration that seems natural.
> There is an exercise here working with the trivial monad W:
> Write a function
> g :: W a -> W a -> W a
> such that
> g (W x) (W y) = W (x+y)
> except don't use pattern matching, but >>= instead. The answer is
> g mx my = mx >>= (\x -> my >>= \y -> W (x+y))
> There are a couple things here that threw me off. One is that I
> didn't expect 'my' to be available inside the first lambda. I
> somehow thought of lambda as isolated, sealed-off from the rest of
> the universe. But they aren't. I believe this is the concept of
> closures, or related to it?
> Secondly, I didn't expect >>= to be available inside the lambda.
> This is related to the mistaken conception of >>= as a procedural
> statement rather than an expression. In Python, where I have
> previously encountered lambdas, no statements are allowed inside
> lambdas. Of course, >>= is actually an expression and you can put
> any expression to the right of a lambda ->.
> Maybe these are typical beginner misconceptions, or maybe they have
> more to do with coming from Python and complete beginners actually
> find it more natural.
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