On 2009 Feb 9, at 20:43, Tom Poliquin wrote:
I'm working on learning arrows. This has led me to ArrowLoop, which then led me to trying to understand the following,
tracePlus b = let (c,d) = (d,b:d) in c
main = do w <- return $ take 10 $ tracePlus 7 print w
1) How does recursion happen?
Being an ex-Schemer I recalled that let x = 3 get translated into something like
(lambda (x) ... ) 3
So now there's a function involved. It's clear that b:d is
(cons b d)
another function. So with appropriate plumbing I could see the thing recurses but the Haskell viewpoint eludes me.
The trick is that the c and d on both sides of the equal sign are identical. Since this asserts that d = b:d, Haskell repeatedly prepends b to d (lazily, so "take 10" halts the recursion). let (c,d) = (d,b:d) in c = d let (c,d) = (b:d,b:b:d) in c = b:d let (c,d) = (b:b:d,b:b:b:d) in c = b:b:d ... As long as something consumes elements from c, that let will continue to expand recursively. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH