RE: [Haskell-cafe] Parse text difficulty

On 09 December 2004 16:37, Malcolm Wallace wrote:

Robert Dockins writes:

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Prelude> [1..5] `zipWith (+)` [7..] <interactive>:1: parse error on input `('

is there a technical reason for this or did it just happen?

If you are asking why general expressions are prohibited between backticks, yes, there is a reason. The expression could be arbitrarily large, so you might have to search many lines to find the closing backtick. But in such a situation, it is surely much more likely that the programmer has simply forgotten to close the ticks around a simple identifier. Just think of the potential for delightfully baffling type error messages that might result!

Forgetting to close a tick would lead to an odd number of ticks, which would probably be flagged as a parse error. Unless you had an even number of missing ticks, of course :-) You can't allow completely general expressions inside backticks, otherwise you run into ambiguity with: 1 `plus` 2 + 3 `plus` 4 so, is that (plus 1 2) + (plus 3 4) or is it ((2+3) plus plus) 1 4 So it would have to be `fexp` only (see the Haskell grammar for fexp). And there's fixities: you can specify the fixity of `x`, but it doesn't make sense to specify the fixity of an expression. There's a historical reason too: `x` and (+) used to be single lexical tokens. I think that changed in Haskell 1.3, but I could be mistaken. Cheers, Simon