[Haskell-cafe] Error Monad and strings

Gerald Gutierrez gerald.gutierrez at gmail.com
Tue Jul 27 19:08:39 EDT 2010

On Tue, Jul 27, 2010 at 3:57 PM, Dietrich Epp <depp at zdome.net> wrote:

> The "strMsg" method is used to implement the "fail" method in the resulting
> method, and calls to "fail" might be inserted into your code even if you
> don't explicitly call it.  An example in GHCi:
> Prelude> :m + Control.Monad.Error
> Prelude Control.Monad.Error> do { Just x <- return Nothing ; return x } ::
> Either String Int
> Left "Pattern match failure in do expression at <interactive>:1:5-8"
> On 2010 July 27, at 15:32, Gerald Gutierrez wrote:
>> Reading the Control.Monad.Error documentation, I see that the Error class
>> has noMsg and strMsg as its only two functions.
>> Now, I understand that you can define your own Error instances such as in
>> example 1 of the documentation, so why the need to always support strings
>> via noMsg/strMsg ? What uses these? And if in my code, I will never throw an
>> error with a string, am I supposed to implement these functions and then
>> ignore them?
I see. So strings must be supported in the case of a bug which cannot be
caught at compile time? In other words, if I get an error with a string, I'm
pretty much guaranteed it is a bug, i.e. a pattern match error as the "fail"
documentation says.
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