This is reminiscent of the Either (exception) monad where Left values, the exceptions, pass through unaltered, and Right values are transformable, i.e. acted on by functions. But I have no idea what you're trying to achieve in the bigger picture. Help us help you by fleshing out your use case. -- Kim-Ee On Sat, Dec 22, 2012 at 12:46 AM, Radical wrote:

Sometimes I'll need something like:

if value == Foo then Bar else value

Or some syntactic variation thereof:

case value of { Foo -> Bar; _ -> value }

Is there a better/shorter way to do it? I'm surprised that it's more complicated to substitute a value on its own than e.g. in a list, using filter. Or perhaps I'm missing the right abstraction?

Thanks,

Alvaro

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Seems like the function is easy to define: replaceIfEq a b c = if c == a then b else c Then the above can be written replaceIfEq Foo Bar value Or the slightly more general (in exchange for slightly more verbosity at the call site) replaceIf p r a = if p a then r else a replaceIf (== Foo) Bar value On Fri, Dec 21, 2012 at 6:46 PM, Radical wrote:

Sometimes I'll need something like:

if value == Foo then Bar else value

Or some syntactic variation thereof:

case value of { Foo -> Bar; _ -> value }

Is there a better/shorter way to do it? I'm surprised that it's more complicated to substitute a value on its own than e.g. in a list, using filter. Or perhaps I'm missing the right abstraction?

Thanks,

Alvaro

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Hi Alvaro, I think you need something wha Scala has - the ability to create a partial function from a case expression. In Scala you could write def update[A](f: PartialFunction[A,A])(v: A): A = f.orElse({ case x => x } : PartialFunction[A,A]).apply(v); and then use it like update[Int]({ case Foo => Bar }) But AFAIK there is nothing like this in Haskell. Maybe separating 'of' from 'case' would be the way to extend Haskell with such a feature < http://www.haskell.org/pipermail/haskell-cafe/2012-November/104884.html> Best regards, Petr 2012/12/21 Radical

Sometimes I'll need something like:

if value == Foo then Bar else value

Or some syntactic variation thereof:

case value of { Foo -> Bar; _ -> value }

Is there a better/shorter way to do it? I'm surprised that it's more complicated to substitute a value on its own than e.g. in a list, using filter. Or perhaps I'm missing the right abstraction?

Thanks,

Alvaro

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

2012/12/21 Radical :

Sometimes I'll need something like:

if value == Foo then Bar else value

Or some syntactic variation thereof:

case value of { Foo -> Bar; _ -> value }

Is there a better/shorter way to do it? I'm surprised that it's more complicated to substitute a value on its own than e.g. in a list, using filter. Or perhaps I'm missing the right abstraction?

Haskell doesn't offer a compact "ternary operator" or similar construct. In some cases, a local definition and pattern guards is appealing: {-# LANGUAGE PatternGuards #-} f value = ... value' ... where value' | Foo <- value = Bar | otherwise = value This does not really have the intuitive of appeal that a "pattern matching ternary operator" might, though: f value = ... (Foo <- value ? Bar : value) ... In Haskell, working with patterns generally is cleaner with multiple lines and full indentation. -- Jason Dusek pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B