Curious demand in a function parameter

25 Mar 2018

      Hi,

In this program

    {-# LANGUAGE MagicHash #-}

    module Lib where

    import Control.Exception
    import GHC.Exts
    import GHC.IO

    data Err = Err
      deriving (Show)
    instance Exception Err

    f :: Int -> Int -> IO Int
    f x y | x > 0     = IO (raiseIO# (toException Err))
          | y > 0     = return 1
          | otherwise = return 2

when I compile this with 8.4 -O2 I get a strict demand on `y`:

    f :: Int -> Int -> IO Int
    [GblId,
     Arity=3,
     Str=,
     ...]

but clearly `y` is not used on all code paths, so I don't understand why we
have a strict demand here.

I found this example in the comments around `raiseIO#`:

    -- raiseIO# needs to be a primop, because exceptions in the IO monad
    -- must be *precise* - we don't want the strictness analyser turning
    -- one kind of bottom into another, as it is allowed to do in pure code.
    --
    -- But we *do* want to know that it returns bottom after
    -- being applied to two arguments, so that this function is strict in y
    --     f x y | x>0       = raiseIO blah
    --           | y>0       = return 1
    --           | otherwise = return 2

However it doesn't explain why we want be strict on `y`.

Interestingly, when I try to make GHC generate a worker and a wrapper for this
function to make the program fail by evaluating `y` eagerly I somehow got a
lazy demand on `y`:

    {-# LANGUAGE MagicHash #-}

    module Main where

    import Control.Exception
    import GHC.Exts
    import GHC.IO

    data Err = Err
      deriving (Show)
    instance Exception Err

    f :: Int -> Int -> IO Int
    f x y | x > 0     = IO (raiseIO# (toException Err))
          | y > 0     = f x (y - 1)
          | otherwise = f (x - 1) y

    main = f 1 undefined

I was thinking that this program should fail with "undefined" instead of "Err",
but the demand I got for `f` became:

    f :: Int -> Int -> IO Int
    [GblId,
     Arity=2,
     Str=,
     ...]

which makes sense to me. But I don't understand how my changes can change `y`s
demand, and why the original demand is strict rather than lazy. Could anyone
give me some pointers?

Thanks

Ömer

Ömer Sinan Ağacan

Sebastian Graf

Simon Peyton Jones

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