Hi Oleg,

just now I wrote a message to haskell-pr...@haskell.org to propose a non-recursive let. Unfortunately, the default let is recursive, so we only have names like let' for it. I also mentioned the ugly workaround (<- return $) that I was shocked to see the first time, but use myself sometimes now.


On 10.07.2013 09:34, o...@okmij.org wrote:
Andreas wrote:
The greater evil is that Haskell does not have a non-recursive let.
This is source of many non-termination bugs, including this one here.
let should be non-recursive by default, and for recursion we could have
the good old "let rec".

Hear, hear! In OCaml, I can (and often do) write

         let (x,s) = foo 1 [] in
         let (y,s) = bar x s in
         let (z,s) = baz x y s in ...

In Haskell I'll have to uniquely number the s's:

         let (x,s1)  = foo 1 [] in
         let (y,s2)  = bar x s1 in
         let (z,s3)  = baz x y s2 in ...

and re-number them if I insert a new statement. BASIC comes to mind. I
tried to lobby Simon Peyton-Jones for the non-recursive let a couple
of years ago. He said, write a proposal. It's still being
written... Perhaps you might want to write it now.

In the meanwhile, there is a very ugly workaround:

     test = runIdentity $ do
      (x,s) <- return $ foo 1 []
      (y,s) <- return $ bar x s
      (z,s) <- return $ baz x y s
      return (z,s)

After all, bind is non-recursive let.

Andreas Abel  <><      Du bist der geliebte Mensch.

Theoretical Computer Science, University of Munich
Oettingenstr. 67, D-80538 Munich, GERMANY


Haskell-Cafe mailing list

Reply via email to