[Haskell-beginners] MonadError

Dennis Raddle dennis.raddle at gmail.com
Sat Aug 25 09:30:23 CEST 2012

I'm enjoying writing code that uses the Error monad to provide errors,
which I can catch and rethrow to add context. Kind of like getting a stack
trace with any context I want every time an error happens.

In most cases I use monad transformers, so the actual monad type would be
something like "ErrorT String (State Int) a"

But much of my code that handles errors doesn't need to know the entire
context. It uses throwError, but otherwise doesn't care anything else about
the type of the monadic computation. I.e., it needs to "know" that it's
inside a monadic computation of a typeclass MonadError.

So I discovered that I could declare a function like this:

x :: MonadError String m => Float -> m Float
x y = if y == 0
  then throwError "zero"
  else return y

My awareness of class constraints is vague, so as I worked on this problem
I knew in a fuzzy way it was going to look something like that, but I had
to experiment.

Also, because I put "String" in the class constraint, I need to turn on

So my question is, how would I do it without making "String" explicit?

If I put

x:: MonadError e m => Float -> m Float
x y = if y == 0
  then throwError "zero"
  else return y

I get errors saying "couldn't match expected type e against inferred type
[Char]" and something about functional dependencies

I fiddled with it a bit but couldn't make it work.
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