Make XNoMonomorphismRestriction the default in GHCi

While it has been a subject of debate whether the Monomorphism Restriction is helpful in compiled code, it is quite clear that MR is a major annoyance, if not a bug, at the GHCi prompt.

MR at the GHCi prompt makes it inconvenient to define functions using let at the prompt. It also unexpectedly assigns monomorphic types to expressions entered interactively by the user - in particular when it types numeric constants as Integer when the user needs an Int.

None of the purported benefits of MR are relevant in GHCi. The kinds of problems that MR might help avoid are unlikely to arise during an interactive session, since the structure of functions entered at the prompt is on average far less complex than functions in source files. Any problems that do arise will become clear as the interactive session continues - likely with a clearer error message (in my personal experience).

MR at the GHCi prompt is one of the most confusing aspects of GHCi for inexperienced users, as evidenced by constant traffic on haskell-cafe and #haskell.

Disabling MR at the GHCi prompt would not be a disruptive change. It would not change the meaning of any program. It would improve the GHCi workflow of inexperienced users, and probably most experienced users. If there are any experienced users who feel that this change disrupts their workflow, they can easily revert to the current behavior by adding a single line to their .ghci file.

For reference, here is a long discussion about MR that took place on haskell-cafe a few years ago:

http://www.haskell.org/pipermail/haskell-prime/2006-January/000038.html

Trac metadata

Trac field	Value
Version	6.10.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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