martin | 29 Oct 22:36 2013
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Getting types inside a function in ghci

Hello all,

is there a way to determine types of symbols which are not toplevel but inside a function?
Christopher Done | 29 Oct 23:06 2013
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Re: Getting types inside a function in ghci

On 29 October 2013 22:36, martin <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

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.
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martin | 29 Oct 23:34 2013
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Re: Getting types inside a function in ghci

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?
Christopher Done | 29 Oct 23:58 2013
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Re: Getting types inside a function in ghci

On the left of <- you could do

do () <- someThing
   ….

so that you'll get a type error against (). Or use scoped type variables:

do (x::()) <- someThing
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Tom Murphy | 30 Oct 04:25 2013
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Re: Getting types inside a function in ghci

FWIW, I asked about this on stackoverflow a while back, and there were a couple of good approaches, although nothing that completely achieves it from ghci. I've used the ghci breakpoint trick a few times, although it's doesn't give the "complete" type.

Here's the link:
http://stackoverflow.com/questions/15034392/find-inferred-type-for-local-function


On Tue, Oct 29, 2013 at 3:06 PM, Christopher Done <chrisdone <at> gmail.com> wrote:
On 29 October 2013 22:36, martin <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

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.

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Gmane