I'm just learning Haskell and I'm still not entirely clear on when and how strict evaluation is forced
When I want a function to evaluate its arguments strictly I find myself writing
((f $! x) $! y ) $! z
which seems weird. Shouldn't $! be left-associative so I could write
f $! x $! y $! z
and have it do what I want?
Am I completely misunderstanding the $! operator?
I found a proposal from 2008 in haskell-prime to make the $ and $! operators left-associative:
https://ghc.haskell.org/trac/haskell-prime/wiki/ChangeDollarAssociativity
There is only one argument against the proposal: "This would break a lot of code".
Instead, there are given four arguments in favour of left-associative ($), the last one being the same as yours, and considered the most important. They are, in short:
0) given the expression f x y, with two applications, we would be able to write f $ x $ y
1) now, with right associative ($), we can write f . g . h $ x as f $ g $ h $ x,
however: \x -> f $ g $ h $ x ==> f $ g $ h is invalid,
so that writing such pipelines with composition is better, as it allows easier cleanup of code
2) Left associative ($) allows you to eliminate more parentheses, in addition to the ones eliminated with (.), for instance:
f (g x) (h y) ==> f $ g x $ h y
3) your argument: the right associative version of $! is inconvenient because of giving rise to things like: ((f $! x) $! y) $! z instead of f $! x $! y $! z
I give support to use the better left-associative version of application operators redefining them at the beginning of our code, like this:
import Prelude hiding (($), ($!))
infixl 0 $, $!
($), ($!) :: (a -> b) -> a -> b
f $ x = f x
f $! x = x `seq` f x
f $! g $! x with the cited tactic for plain $ , one would need some sort of strict composition: f .! g $! x. I wonder: is there such operator already defined in a library? –
Jere <$> is consistent with the current conventional $: f <$> g <$> x seems to be equivalent to f . g <$> x. But another, left-associative version of <$> wouldn't make much sense, because consuming several Applicative arguments is written as f <$> x <*> y or g <*> x <*> y (for a pure f or an Applicative g). It seems that <*> is already left-associative (as wanted for $ in Haskell-prime), and lower priority than <$>. –
Jere <$> is left-associative in the latest base (so, inconsistent with the conventional $), contrary to what I said in the above comment. But my <$> from an older base I have on my system is right associative! (show <$> show <$> Just 3 gives Just "\"3\"" for me; or I don't understand what is happening, so I asked a question) –
Jere (.!): Strict composition. –
Jere It's to mirror the fixity of $. You could make a very good case for both $ and $! having the wrong fixity.
*%^*%^*&&$&%^ a Haskell operator? Well, it could be, if some library defined it. –
Etymon I found a proposal from 2008 in haskell-prime to make the $ and $! operators left-associative:
https://ghc.haskell.org/trac/haskell-prime/wiki/ChangeDollarAssociativity
There is only one argument against the proposal: "This would break a lot of code".
Instead, there are given four arguments in favour of left-associative ($), the last one being the same as yours, and considered the most important. They are, in short:
0) given the expression f x y, with two applications, we would be able to write f $ x $ y
1) now, with right associative ($), we can write f . g . h $ x as f $ g $ h $ x,
however: \x -> f $ g $ h $ x ==> f $ g $ h is invalid,
so that writing such pipelines with composition is better, as it allows easier cleanup of code
2) Left associative ($) allows you to eliminate more parentheses, in addition to the ones eliminated with (.), for instance:
f (g x) (h y) ==> f $ g x $ h y
3) your argument: the right associative version of $! is inconvenient because of giving rise to things like: ((f $! x) $! y) $! z instead of f $! x $! y $! z
I give support to use the better left-associative version of application operators redefining them at the beginning of our code, like this:
import Prelude hiding (($), ($!))
infixl 0 $, $!
($), ($!) :: (a -> b) -> a -> b
f $ x = f x
f $! x = x `seq` f x
f $! g $! x with the cited tactic for plain $ , one would need some sort of strict composition: f .! g $! x. I wonder: is there such operator already defined in a library? –
Jere <$> is consistent with the current conventional $: f <$> g <$> x seems to be equivalent to f . g <$> x. But another, left-associative version of <$> wouldn't make much sense, because consuming several Applicative arguments is written as f <$> x <*> y or g <*> x <*> y (for a pure f or an Applicative g). It seems that <*> is already left-associative (as wanted for $ in Haskell-prime), and lower priority than <$>. –
Jere <$> is left-associative in the latest base (so, inconsistent with the conventional $), contrary to what I said in the above comment. But my <$> from an older base I have on my system is right associative! (show <$> show <$> Just 3 gives Just "\"3\"" for me; or I don't understand what is happening, so I asked a question) –
Jere (.!): Strict composition. –
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!$with the reversed fixity of$!that does what you want? – Evanish