Brent Yorgey
On Sat, Feb 28, 2009 at 05:19:03PM +0000, Will Ness wrote:
writes: Hello.
Is there any non-recursive binding expression in Haskell?
Something that would allow me to write, for instance,
test = let' xs = [] in let' xs = 3 : xs in let' xs = 8 : xs in let' xs = 7 : xs in xs
Well, it's not a nice thing to do (probably), but you can write
test = head $ do xs <- [ [] ] xs <- [ 3:xs ] xs <- [ 8:xs ] xs <- [ 7:xs ] return xs
The correct answer is no. Life is pain. Anyone who says otherwise is selling something. ;)
Assignment in Haskell is not destructive update---it assigns a name to a value, and they become one flesh, until parted by death (i.e. the end of their lexical scope). The only reason Will's code works is that each line creates a new lexical scope, and each xs shadows the previous one.
That's what I understood the OP wanted - Scheme's LET, not LETREC, allowing for shadowing. I was suprised let-statement in do chain didn't work that way. I expected it to be equivalent to a kind of code above, since each new line in do block represents a nested function in explicit bind notation, and nested function binding definitely provides for non-recursive let kind of argument binding, with shadowing. I thought the whole point of having special let statement in do notation was not to have to write the kind of code above with singleton lists. Since we have shadowing there, it should've been so in let-statements too. Isn't it?
To do what you want, you have to give each thing a new name, something like this:
test = let xs = [] xs' = 3 : xs xs'' = 8 : xs' xs''' = 7 : xs'' in xs'''
"But this is horribly painful!" you cry. Of course it is! Giving names to things you don't want to keep is always painful (like when your child names the almost-dead raccoon you find in the street, which is just going to die or be given to an animal shelter anyway).
So, why not just avoid naming things you don't want?
test = (7:) . (8:) . (3:) $ []
Ah, that's better! We just thread some intermediate values through a chain of composed functions. Each function does its little bit of modification and passes the intermediate value along to the next function, and the intermediate values are never given names.
-Brent
BTW could there be a use for something like infixl 1 # x # f = f x -- (#) = flip ($) to have the direct data flow reflected in our code, so that your code would become test = [] # (3:) # (8:) # (7:) maybe sometimes it's more natural to think of data being "piped through" the chain of functions, and to write them down in forward, not reverse order of application?