[Haskell-cafe] Getting types inside a function in ghci

martin martin.drautzburg at web.de
Tue Oct 29 22:34:21 UTC 2013

Am 10/29/2013 11:06 PM, schrieb Christopher Done:
> On 29 October 2013 22:36, martin <martin.drautzburg at web.de <mailto:martin.drautzburg at web.de>> wrote:
>     is there a way to determine types of symbols which are not toplevel but inside a function?
> Not at present without transformation. There are two reasonably reliable approaches:
> Insert noisy holes/make intentional type errors: http://ghc.haskell.org/trac/ghc/wiki/Holes#Insertingdeliberatetypeerrors
> Implicit parameters: http://ghc.haskell.org/trac/ghc/wiki/Holes#Implicitparameters
> Holes will land in GHC to support this explicitly. I don't know what version of GHC has/will have this.
> Alternatively, you can use ghc-mod (https://github.com/kazu-yamamoto/ghc-mod) which, if you can get it to work, can tell
> you the types of sub-expressions.
> Lastly, the next version of the FP Complete IDE gives you types of sub-expressions just by clicking on them or selecting
> them out of the box.

Thanks, that helps.

Related questions: how can I figure out the type (using holes) for the symbol left of the <- in a do expression? I mean
other than:

main = do
    x <- getChar::undefined
    return ()

Couldn't match type `undefined' with `IO Char'

And how about plain lambdas. This works

f = \x -> (x::undefined)+1

Couldn't match type `t' with `undefined'

but what if x does not appear on the right side?

More information about the Haskell-Cafe mailing list