Proposal: Non-recursive let

[Andreas Abel]

Proposal: add a non-recursive let to the Haskell language.  In

   let' p = e in e'
   do { ... let' p = e ... }

the variables of pattern p are then *not* in scope in e.

Reasons for adding a non-recursive let:

1. recursive-let is the source for many non-termination bugs.

An instance of such a bug is in the state monad of mtl-2.1 (forgive me, 
Edward, for quoting this again):

 
http://hackage.haskell.org/packages/archive/mtl/2.1/doc/html/src/Control-Monad-State-Class.html#state 


     state :: (s -> (a, s)) -> m a
     state f = do
       s <- get
       let ~(a, s) = f s
       put s
       return a

here, a non-recursive let was meant, the s bound by the let was supposed 
to be a new state, shadowing the old state.  However, since let is 
recursive, this actually causes a loop.

Another instance (cut-down) are let-guards like

   let Just x | x > 0 = e in x

The "x > 0" is understood as an assertion here, documenting an 
invariant.  However, Haskell reads this as

   let Just x = case () of { () | x > 0 -> e } in x

leading to non-termination.  A non-recursive version

   let' Just x | x > 0 = e in x

could be interpreted as

   case e of { Just x | x > 0 -> x }

instead, which is meaningful (in contrast to the current interpretation).

2.  Connected to 1., a non-recursive let allows for painless shadowing.

For instance, the hack

   do ...
      x <- return $ f x

exploiting non-recursiveness of the <- binding, would be unnecessary, 
instead we could write

   do ...
      let' x = f x

NB: In a do-Block, we could drop the keyword let' and simply write

   do ...
      x = f x

for a non-monadic binding, in accordance to x <- mf x,

See also

   http://www.haskell.org/pipermail/beginners/2009-February/001075.html
 
http://stackoverflow.com/questions/8398607/declaration-order-in-let-bindings-haskell-vs-ocaml

PS.:  I can work out the details of grammar and semantics if this 
proposal finds followers.

Cheers,
Andreas

-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

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

andreas.abel at ifi.lmu.de
http://www2.tcs.ifi.lmu.de/~abel/