[Haskell-beginners] "stage restriction" when using Data.Heap

[Darren Grant]

The GHC stage restriction is an error invoked when some Template Haskell
[1] construct requires the evaluation of run-time values at compile time. I
am going out on a limb and assuming that same code probably works in GHCI
because the interpreter does not have a discrete compile-time phase.

I assume that MinHeap is templated, but exactly how I do not know. Maybe
someone who is more familiar with Data.Heap will have an answer.

[1] http://www.haskell.org/haskellwiki/Template_Haskell




On Mon, Jan 7, 2013 at 10:03 PM, Martin Drautzburg <Martin.Drautzburg at web.de
> wrote:

> When I :load this into ghci
>
> import Data.Heap
>
> heap = empty :: MinHeap (Int,String)
> insert (1,"kjh") heap
>
> I get
>
>     GHC stage restriction: `heap'
>       is used in a top-level splice or annotation,
>       and must be imported, not defined locally
>     In the second argument of `insert', namely `heap'
>     In the expression: insert (1, "kjh") heap
>
> But when I type in
>
> import Data.Heap
>
> let heap = empty :: MinHeap (Int,String)
> insert (1,"kjh") heap
>
> everything works and I get
>
>  fromList [(1,"kjh")]
>
> Why is that so and what can I do to prevent the "stage restriction"?
> --
> Martin
>
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