[Haskell] Mixing monadic and non-monadic functions

Bjorn Lisper lisper at it.kth.se
Wed Sep 14 16:10:39 EDT 2005

Ben Rudiak-Gould:
>Frederik Eaton wrote:
>> I want the type system to be able to do "automatic lifting" of monads,
>> i.e., since [] is a monad, I should be able to write the following:
>> [1,2]+[3,4]
>> and have it interpreted as "do {a<-[1,2]; b<-[3,4]; return (a+b)}".
>The main problem is ambiguity: [[1]]++[[2]] could be [[1],[2]] or [[1,2]], 
>for example. If your proposal resolves this ambiguity in favor of one result 
>or the other, then I'm opposed to it -- it's far too easy in practice to 
>write code like this, expecting it to be lifted, and failing to notice that 
>it also has an interpretation without lifting (or the other way around). 
>This is the Perl FWIM disease[1] -- not that I dislike Perl, but people are 
>quite rightly leery about bringing this sort of thing into Haskell.

However, there is a way to resolve the ambiguity that can be claimed to be
the most natural one, and that is to always choose the "least possible"
lifting. In the example above, this would mean to interpret [[1]]++[[2]]
precisely as [[1]]++[[2]] (lift 0 levels) rather than [[1]++[2]] (lift 1
level). This is akin to choosing the most general type in the pure
Hindley-Milner type system, and it has the advantage that expressions that
are typable in the original type system, without lifting, retain their
semantics in the type system with lifting added.

Lifting (mainly of arithmetic operators) has been around for a long time in
array- and data parallel languages such as Fortran 90 and *lisp.  The kind
of ambiguity mentioned here was first observed for nested data-parallel
languages like NESL, which use nested sequences as parallel data structures.

Now, whether to include this kind of lifting in Haskell or not is an
entirely different story. One complication would be to handle possible
clashes between lifting and overloading through the class system. IMHO, I
think it would be interesting to be able to define application-specific
Haskell dialects, which can have this kind of lifting for a restricted set
of types and/or functions, whereas having it as a general feature of the
language would be quite problematic.

Björn Lisper

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