Re: [Haskell-cafe] What does "1 = 2" mean in Haskell?

It seems weird however that Haskell allows let 1 = 0 but does not allow let (f x, g y z) = (x*x, y*z) Alexey. On Friday, February 24, 2017 at 4:53:59 AM UTC+1, Brandon Allbery wrote:

Without a binding it is useless at top level, but if you strictify the pattern it can be useful in `let` (possibly as a sanity check where you want the program to abort if it fails). I don't recall offhand if it desugars usefully in list comprehensions, but if so it would work as a filter. There may also be other specialized use cases; general syntax tends to get reused a lot in Haskell, so making this case a syntax error could make it difficult to support actually useful cases. :)

(Also I'm sure someone overly clever could figure out some way to abuse it. :)

On Thu, Feb 23, 2017 at 10:41 PM, Harendra Kumar javascript:> wrote:

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On 24 February 2017 at 08:45, Brandon Allbery javascript:> wrote:

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Literally the only use I've seen for this was a CCC puzzle. However, it is the trivial case of something that is more useful: pattern matching the result of an expression (say, a Data.Map.lookup when you know the key exists).

Can you explain how that will be useful (without a binding)? Will the pattern match be ever actually tried when there is no binding?

-harendra

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