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Simon Peyton Jones authored
The change is this (see Trac #2798). Instead of writing mdo { a <- getChar ; b <- f c ; c <- g b ; putChar c ; return b } you would write do { a <- getChar ; rec { b <- f c ; c <- g b } ; putChar c ; return b } That is, * 'mdo' is eliminated * 'rec' is added, which groups a bunch of statements into a single recursive statement This 'rec' thing is already present for the arrow notation, so it makes the two more uniform. Moreover, 'rec' lets you say more precisely where the recursion is (if you want to), whereas 'mdo' just says "there's recursion here somewhere". Lastly, all this works with rebindable syntax (which mdo does not). Currently 'mdo' is enabled by -XRecursiveDo. So we now deprecate this flag, with another flag -XDoRec to enable the 'rec' keyword. Implementation notes: * Some changes in Lexer.x * All uses of RecStmt now use record syntax I'm still not really happy with the "rec_ids" and "later_ids" in the RecStmt constructor, but I don't dare change it without consulting Ross about the consequences for arrow syntax.
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