Summary: Haskell layout has problems within 'do' (see below). Can we change Haskell to improve it? We teach Haskell to all our second-year students, and we like to teach the OPTION of using explicit braces and ;'s. (If students write nested do constructs, even though we discourage this, they often have trouble with the layout rules.) We would like to be able to teach this: "To use explicit structuring, instead of layout, you can just put braces around all the statements in a 'do' block, plus semicolons at the end of each statement. (like in C++/Java!)." But this does not work. For example, the first definition below parses, but the second does not. fa::IO () fa = do putStr "Enter Data: " line <- getLine let line2 = line putStr line2 fb::IO () fb = do { putStr "Enter Data: "; line <- getLine; let line2 = line; putStr line2; } ERROR "f.hs" (line 13): Syntax error in definition (unexpected symbol "putStr") Fix 1: Put explicit braces around the body of the 'let': let {line2 = line}; This is ugly and a nuisance to teach. It would be much nicer to be able to teach 'let pattern = expr' as a simple statement, and avoid mentioning the possibility of multiple definitions within a single 'let'. (Which is rarely needed within 'do' syntax anyway, because you can usually use several separate let statements instead. Mutually recursive definitions is probably the only reason.) Fix 2: an alternative fix is to move the semicolon at the end of the let to the beginning of the next line, but this is ugly! f2 = do { putStr "Enter Data: "; line <- getLine; let line2 = line ;putStr line2 } Fix 3: I might be happy to put ALL the semicolons at the beginning of lines, which also works, but you are not allowed a semicolon at the beginning of the first brace, so this looks asymmetric. f3 = do { putStr "Enter Data: " ;line <- getLine ;let line2 = line ;putStr line2 } Question: Is it possible to change the layout handling of Haskell slightly so that f1 is legal? Perhaps the explicit semicolon at the end of the let line could terminate the (implicit) layout of the let statement? Mark. Dr Mark Utting, Senior Lecturer Department of Computer Science School of Computing and Mathematical Sciences The University of Waikato Tel: +64 7 838 4791 Private Bag 3105 Fax: +64 7 858 5095 Hamilton Email: marku@cs.waikato.ac.nz New Zealand Web: http://www.cs.waikato.ac.nz/~marku It is a time-proven principle of software engineering to try and strengthen the static description of systems as much as possible. [Clemens Szyperski]