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@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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