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I've been playing with some examples from Simon Marlow's book about parallel and concurrent programming in Haskell and stumbled across an interesting behavior that I don't really understand. This is really about me trying to understand some of the inner workings of GHC.

Let's say I do the following in the REPL:

λ» let x = 1 + 2 :: Int
λ» let z = (x,x)
λ» :sprint x
x = _
λ» :sprint z
z = (_,_)
λ» seq x ()
()
λ» :sprint z
z = (3,3)

Ok, this is pretty much what I expected except that z gets evaluated to WHNF already. Let's write a similar program and put it in a file:

module Thunk where

import Debug.Trace

x :: Int
x = trace "add" $ 1 + 2

z :: (Int,Int)
z = (x,x)

And fiddle around with it in GHCi:

λ» :sprint x
x = _
λ» :sprint z
z = _
λ» seq x ()
add
()
λ» :sprint z
z = _
λ» seq z ()
()
λ» z
(3,3)

So this behaves a little different: z is not evaluated to WHNF in advance. My question is:

Why is z evaluated to WHNF in the REPL when doing let z = (x,x) but not when loading the definition from a file. My suspicion is that it has something to do with pattern binding but I don't know where to look that up for clarification (maybe I'm just completely utterly wrong). I would have expected it to somehow behave like the example in the file.

Any pointers or a brief explanation why this happens?

Hossein Narimani Rad
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raichoo
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    Because `(,)` is a constructor, the difference makes no difference to Haskell's semantics (`:sprint` gives access to internal thunk implementation details so doesn't count.) So this is a question of which optimizations and trade-offs GHC does when compiling `(x,x)` in different positions. Someone else may know the precise reason in these cases. – Ørjan Johansen Jul 15 '14 at 10:33
  • @ØrjanJohansen I feel like your comment should be an answer. – codygman Mar 02 '15 at 23:16
  • @Codygman If you think so. I don't really know more about it, though, so I had sort of been hoping for "someone else" to show up. – Ørjan Johansen Mar 03 '15 at 07:19

1 Answers1

2

Because (,) is a constructor, the difference makes no difference to Haskell's semantics (:sprint gives access to internal thunk implementation details so doesn't count.) So this is a question of which optimizations and trade-offs GHC does when compiling (x,x) in different positions. Someone else may know the precise reason in these cases.

Ørjan Johansen
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