[Haskell-cafe] A better syntax for qualified operators?

Brandon Moore brandonm at yahoo-inc.com
Wed Sep 27 18:02:25 EDT 2006

Brian Hulley wrote:
> Hi -
> Consider the scenario when you want to find a function that returns 
> the i'th element of an array but all you know is that there is a 
> module called Data.Array.IArray that will probably have such a 
> function in it. So you start typing in your program:
>    let
>        ith = Data.Array.IArray.
> at this point, you'd hope the editor you're using would somehow 
> display a list of avaliable values exported from Data.Array.IArray 
> including the indexing function, so you could select it, thus I would 
> *like* to be able to use the syntax:
>    let
>        ith = Data.Array.IArray.(!)
> because it's not the user's fault that the person who wrote 
> Data.Array.IArray decided to use a symbol instead of an identifier for 
> this function - the user of Data.Array.IArray in this case just wants 
> to see normal identifiers to use with prefix application so the use of 
> (!) at this point effectively gets rid of the unwanted operatorness 
> associated with the function.

This is a nice argument

> However the current syntax of Haskell would not allow this. Instead 
> you have to write:
>    let
>        ith = (Data.Array.IArray.!)
> The problem is that the user of Data.Array.IArray has to know already 
> in advance, before typing the 'D' of "Data", that the indexing 
> function has been named with a symbol instead of an identifier, but 
> this knowledge is only available later, when the user has typed the 
> '.' after "IArray", so the current syntax would be frustrating for the 
> user because the user then has to go all the way back and insert an 
> opening paren before the 'D'.
> Also, consider the appearance of:
>    let
>        ith = (Data.Array.IArray.!) arr i
>        b = Data.Array.IArray.bounds arr
> vs
>    let
>        ith = Data.Array.IArray.(!) arr i
>        b = Data.Array.IArray.bounds arr
> I'm not sure if I've managed to explain this problem clearly enough, 
> but my proposal is that we might consider changing the lexical syntax 
> of Haskell as follows:
>    varId ::= id
>    varOp ::= symbol
>    varIdOp ::= ` varId
>    varOpId ::= ( varOp )
>    varOpIdOp ::= ` varOpId
>    qvarId ::= {conId .}+ varId    -- { }+ denotes 1 or more times
>    qvarIdOp ::= ` qvarId
>    qvarOp ::= {conId .}+ varOp
>    qvarOpId ::= {conId .}+ varOpId
>    qvarOpIdOp ::= `qvarOpId
> In other words, to turn an operator symbol into an id, the parentheses 
> would be put immediately around the symbol (with no spaces since this 
> is lexical syntax), and to turn an id into an operator the backquote 
> is put in front of the entire (qualified) id.
Why does the nice argument not apply equally well to infixifying things?

If I think I want to use infix some thing from Data.Array
and start typing

myArr Data.Array.

and find out element access has become "get" while I wasn't looking, 
it's not my fault the author of Data.Array decided to use a function 
when I was expecting an identifier - Shouldn't I be able to write

myArr Data.Arr.`get` ix

> (Also the trailing backquote in the existing syntax is redundant)

The trailing backquote is just as redundant as the trailing close paren 
in the syntax for using a symbol as a prefix function and just as 
important for my comment on backticks as the closing paren is to your 
proposal for sections -
it means it's lexically apparent at least at one side of the identifier 
that it's a section/infixification


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