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I'm still trying to get a hang of the parallels between arrow notation and the semantics of the Arrow typeclasses defined in Haskell. In particular, this question seems to have a very canonical example of a small counter written with arrow notation:
counter :: ArrowCircuit a => a Bool Int
counter = proc reset -> do
rec output <- returnA -< if reset then 0 else next
next <- delay 0 -< output+1
returnA -< output
Can someone show me how to convert this back into Haskell2010 without arrow notation?
522
It's a new feature that introduced in ES6 and is called arrow function. The left part denotes the input of a function and the right part the output of that function.
What It Is. This is an arrow function. Arrow functions are a short syntax, introduced by ECMAscript 6, that can be used similarly to the way you would use function expressions. In other words, you can often use them in place of expressions like function (foo) {...} .
You can set the return type of an arrow function in TypeScript right after its parameters, e.g. const greet = (name: string): string => {} . Once a function's return type is set, the type checker alerts us if the function returns a value of a different type.
ES6 version of TypeScript provides an arrow function which is the shorthand syntax for defining the anonymous function, i.e., for function expressions. It omits the function keyword. We can call it fat arrow (because -> is a thin arrow and => is a "fat" arrow). It is also called a Lambda function.
{- |
+---------+
>Bool>--------------> |
| >------------------>Int>
+---------+ | arr f |
/----> delay 0 >---> >---------\
| +---------+ | | |
| +---------+ |
| |
\--------------------------------------/
-}
counter' :: ArrowCircuit a => a Bool Int
counter' = loop $ second (delay 0) >>> arr f
where
f (reset, next) = let output = if reset then 0 else next
next' = output + 1
in (output, next')
The recursive rec part is implemented using loop. The inner part that converts reset to output using next (and producing new next value) is just a pure function with two inputs and two outputs.
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