On Sep 17, 2009, at 20:17 , xu zhang wrote:

   case op of
         And -> let a = 4
         Or -> let a = 3
         Implies -> let a = 2
         Equiv   -> let a = 1

"let" isn't an assignment command, it's a scoping command. That is,

> let a = 3 in ...

is equivalent to something like

{ int a = 3; ... } // except the type can be inferred

Note that the "in" is required unless part of a do-block, where block structure is implied.

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