[Haskell-cafe] Proposal: Applicative => Monad: Call for consensus

Casey Hawthorne caseyh at istar.ca
Sat Jan 22 04:58:36 CET 2011

uj supplied this:

About the discussion 
"putStrLn (readLn + (5 :: Int))".. 

I'll write it as the following line, 

importing Control.Applicative
main = (+) readLn (return 3)

They look almost exactly same in my eyes..

On Fri, 21 Jan 2011 11:01:36 -0800, you wrote:

>Interesting little paper, Tyson.
>You bring up other programming languages and 'ad-hoc systems for
>resolving ambiguities'; I agree with you that these systems generally
>have no strong theoretical basis, but I'm not sure that's a terribly
>bad thing.
>I think what a programmer actually wants from ambiguity resolution is
>something *predictable*; C++'s system is definitely stretching the
>boundaries of predictability, but any case where I have to break out a
>calculator to decide whether the compiler is going to choose
>specification A or specification B for my program seems like a
>failure.  I'd much rather the solution wasn't always 'the most
>probable' but at least was easy for me to figure out without thinking
>too hard.
>The goal is to easily know when I have to manually specify ambiguity
>resolution and when I can trust the compiler to do it for me.  I
>didn't completely follow the math in your paper, so maybe it turns out
>simply if it was implemented, but it wasn't clear to me.  At the
>least, I think you should add examples of the types of ambiguity
>resolution you'd like the compiler to figure out and what your
>probability measure chooses as the correct answer in each case.
>Anyways, thanks for the interesting read.  I'm excited to see work on
>making a better type *inference* system, since much of the work lately
>seems to be on making a better *type* system at the cost of more often
>manually specifying types.
>I work in a traditional programming industry, and most of the people
>from work that I talk to about Haskell are frustrated that they can't
>just write putStrLn (readLn + (5 :: Int)) and have the compiler figure
>out where the lifts and joins go.  After all, that just works in C[1]!
> What's the point of having the most powerful type system in the
>universe if the compiler can't use it to make your life easier?
>  -- ryan
>[1] sample program:
>int readLn(); // reads a line from stdin and converts string to int
>void putStrLn(int x); // prints an int to stdout
>void main() { putStrLn(readLn() + 5); }
>On Fri, Jan 21, 2011 at 8:43 AM, Tyson Whitehead <twhitehead at gmail.com> wrote:
>> On January 19, 2011 15:28:33 Conor McBride wrote:
>>> In each case, the former has (++) acting on lists of strings as pure
>>> values,
>>> while the latter has (++) acting on strings as values given in
>>> []-computations.
>>> The type [String] determines a domain, it does not decompose uniquely
>>> to a
>>> notion of computation and a notion of value. We currently resolve this
>>> ambiguity by using one syntax for pure computations with [String] values
>>> and a different syntax for [] computations with String values.
>>> Just as we use newtypes to put a different spin on types which are
>>> denotationally the same, it might be worth considering a clearer (but
>>> renegotiable) separation of the computation and value aspects of types,
>>> in order to allow a syntax in which functions are typed as if they act
>>> on
>>> *values*, but lifted to whatever notion of computation is ambient.
>> Yes. ?That makes sense. ?Thank you both for the clarification. ?The idea of
>> explicitly separating the two aspects of types is an interesting one.
>> The automated approach I had been thinking of was to always take the simplest
>> context possible. ?(i.e., for the above, list of strings as pure values).
>> To this end I've been working on a measure for the complexity of the
>> application operator. ?I've got a draft at
>> http://www.sharcnet.ca/~tyson/haskell/papers/TypeShape.pdf
>> I'm still working on my thinking on polymorphic types though, so everything
>> from section 2.2 onwards is subject to change (especially 2.3 and the
>> conclusion).
>> Cheers! ?-Tyson
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