Proposal: System.Environment.getEnv should return Maybe

John Lato jwlato at
Thu Feb 23 13:18:08 CET 2012

> From: Jon Fairbairn <jon.fairbairn at>
> Brandon Allbery <allbery.b at> writes:
>> On Wed, Feb 22, 2012 at 18:51, John Meacham <john at> wrote:
>>> On Wed, Feb 22, 2012 at 3:05 PM, Evan Laforge <qdunkan at> wrote:
>>> > It's awkward and surprising how getEnv throws an exception when an env
>>> > var is not set, since it's very common for env vars to be optional,
>>> > and it's not convenient to catch exceptions in haskell (I have to look
>>> For true IO exceptions, you should be using the Prelude.catch mechanism.
>> This still assumes that it's an exceptional condition that is being
>> modeled.  It is not; it is correctly modeled as a Maybe.
> +1. And it?s always irritated me that read at EOF raises an
> exception. Files having ends is not exceptional! (This is partly
> addressed by iteratees, I imagine, but I haven?t found time to
> get my head round those yet.)

This irritates me too; I don't think that Haskell exceptions should be
used for much of anything, exceptional condition or not.

+1 for the proposal.

Also, the unix-bytestring package
( exposes IO
functions that don't raise exceptions at EOF.

John L.

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