scope all wrong? in Mathematica 4.1

*To*: mathgroup at smc.vnet.net*Subject*: [mg31827] scope all wrong? in Mathematica 4.1*From*: Richard Fateman <fateman at cs.berkeley.edu>*Date*: Wed, 5 Dec 2001 06:51:53 -0500 (EST)*Organization*: University of California, Berkeley*Sender*: owner-wri-mathgroup at wolfram.com

Consider the mathematica definition rr[x_] := Block[{x}, x = 5; Print["x is ", x]] what do you expect here? rr[z] --> x is 5 is printed. but you might not expect rr[4] --> "Block::"lvsym": "Local variable specification{4} contains 4 which is not a symbol or an assignment to a symbol." If you use Module instead of Block, you get the same. However, Block[{x = 4}, Block[{x}, x = 5; Print["x is ", x]]] prints x is 5 Using pattern matching for substituting 4 for x in rr[4] even when x is bound seems to be counter to what most programming language designers would expect. Now look at uu[x_?((x = 5; Print["x is ", x]; True) &)] := x Usually when one defines a program, nothing is printed. Here, we get x is 5 5 x is 5 If we do this: x =400 uu[70] x is 5 printed the value returned is 70 and the global x is 5. Try this: Module[{x}, uu[x_?((x = 5; Print["x is ", x]; True) &)] := x; Print["in Module, x is " , x]] The expectation in a system supporting lexical scope with Module is that no use of x inside the module would escape. But it does. The global x is set to 5. The reason this all came up is in correspondence suggesting that programs in one computer algebra system could be translated into another. If systems are semantically "surprising", it is more difficult. I wonder how much of mathematica internally depends on wrong scope, or how much of the code is susceptible to bugs because of unexpected capture of names. (I suspect that this has caused a proliferation of package names e.g. MySecretNameSpace`x in Mathematica routines). RJF