On 11 April 2018 at 06:48:49, Matthias Felleisen (matth...@felleisen.org) wrote:
> Perhaps the real problem is one of the contract/type system. We have seen 
> effect systems  
> over and over again, though usually they try to express complicated 
> invariants and have  
> them checked at compile time. What if contract and type systems came with two 
> arrows:  
> ->! for imperative functions, as in “this function may mutate the given 
> argument”
> -> for ‘pure’ functions, as in “this function promises not to mutate the 
> given argument"  

I think this would be quite helpful. We can then easily enforce invariants like 
“the client-supplied function doesn’t mutate the given argument”. (Is there an 
existing way to enforce this?)

On the other hand, I still like returning #<void> for such imperative 
functions, because: (1) functionality-wise the returned value would be 
redundant; (2) for fluent DSLs, it could be ambiguous which input argument 
should be returned, and the best choice may depend on how the API is used in 
the client code.

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