Re: [Haskell-beginners] $ versus .

The difference is that one of them is: — unnecessary parentheses for emphasis (init tail) [1, 2, 3] And the other is — parentheses required for correct evaluation init (tail [1, 2, 3]) On Tue, Jan 26, 2021 at 08:23 Lawrence Bottorff wrote:

...

:t (init tail) : error: : * Couldn't match expected type `[a]' : with actual type `[a0] -> [a0]' : * Probable cause: `tail' is applied to too few arguments : In the first argument of `init', namely `tail' : In the expression: (init tail)

...

:t (init . tail) : (init . tail) :: [a] -> [a]

...

:t init $ tail : error: * Couldn't match expected type `[a]' with actual type `[a0] -> [a0]' * Probable cause: `tail' is applied to too few arguments In the second argument of `($)', namely `tail' In the expression: init $ tail

...

chopEnds = init $ tail chopEnds [1,2,3] error: ... * Variable not in scope: chopEnds1 :: [Integer] -> t ...

but then

...

init $ tail [1,2,3] [2]

Not sure what I'm missing here. It doesn't make sense to me that the last expression works, but no version of a closure

chopEnds = init $ tail

does.

On Mon, Jan 25, 2021 at 7:58 PM Kim-Ee Yeoh wrote:

...

init $ tail [1,2,3] = init (tail ([1,2,3])) -- a la Lisp

Now, functional programming is awesomest at abstractions. What if we could abstract out "init (tail"?

Then we could write

chopEnds = init (tail

But that looks weird. It's only got the left half of a parens pair!

Does that explain why you should not expect the same result?

A separate question is why the compiler even type-checks "init $ tail" in the first place. What do you think is going on there?

On Tue, Jan 26, 2021 at 1:16 AM Lawrence Bottorff wrote:

...

I've got this

...

init $ tail [1,2,3] [2]

and this

...

chopEnds = init $ tail chopEnds [1,2,3] [1,2]

What happened? Why is it not just init $ tail [1,2,3] ?

This works fine

...

chopEnds2 = init . tail chopEnds2 [1,2,3] [2]

What am I missing?

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-- -- Kim-Ee _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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