[Haskell-cafe] Displaying infered type signature of 'offside' functions

[Derek Elkins]

kahl at cas.mcmaster.ca wrote:
> Jules Bean <jules at jellybean.co.uk> wrote,
> concerning the problem of getting at the types of local definitions:
> 
>  > Simon Peyton-Jones wrote:
>  > The principal difficulties here are to do with "what do we want"
>  > rather the implementation challenges.
>  >
>  > > 1.  Should the compiler print the type of every declaration? Should GHCi allow you to ask the type of a local decl?
>  > >   
>  > 
>  > IMO, ghci should definitely allow you to ask. This comes up for me every 
>  > time that I write any haskell code (and in general I end up hoisting 
>  > local definitions to the top level, which is a real pain if there is 
>  > local scope, data or type, to hoist with it).
>  > 
>  > > 2.  How should the variables be identified?  There may be many local bindings for 'f', so you can't say just ":t f".  Ditto if dumping all local bindings.
>  > >
>  > >   
>  > 
>  > I think this is a hard question. I was imagining some kind of 
>  > hierarchical system like foo.f, in the case that f is locally defined 
>  > inside foo. (Yes I know we can't easily use '.' for that). There might 
>  > be might be multiple fs inside the definition of foo; indeed there might 
>  > even be multiple fs nested inside each other.
> 
> 
> I just wanted to contribute a PRACTICAL TRICK I use:
> 
> 
> * If the local definition is a pattern binding
> 
>     f = ...
> 
>   then I just add
> 
>     f :: Ordering
> 
> * If the local definition is a, say binary, function binding
> 
>     f p1 p2 = ...
> 
>   then I just add
> 
>     f :: Float -> Double -> Ordering
> 
>   (Type does not matter for the number of function arrows you need to put;
>    only the syntactic arity of the bindings matters here.)
> 
> 
> This relies on the fact that the types Float, Double, and Ordering
> very rarely occur in my code --- pick your own.

Why not just declare a type you don't use?