Version 3 (modified by maeder, 5 years ago) (diff) |
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# Proposal: PrefixMinusResolution

Ticket | #138 |

Dependencies | reject NegativeSyntax NegationBindsTightly |

Related | FixityResolution |

## Compiler support

GHC | [ none ] |

nhc98 | [ none ] |

Hugs | [ partial ] |

UHC | [ unknown ] |

JHC | [ none ] |

LHC | [ unknown ] |

## Summary

Resolve more prefix minus application unambiguously by:

- Considering only operators to the right of prefix negation.

- Leave prefix minus bind less tight than multiplication.

- Do not consider associativity of prefix minus or (alternative formulation) let prefix minus bind a bit stronger than infix minus.

## Description

As described in NegationBindsTightly ghc rejects some terms
that are accepted by Hugs and Helium and should not be rejected in general.
However, making negation bind more tightly is not the proposed solution here, because
it would (in my eyes) wrongly resolve "`- x ^ 2`" to "`(-1) ^ 2`".

- Only operators to the right of prefix negation need to be considered for resolution.

This condition ensures that "4 * -5" is not rejected (as ghc does).

- Prefix minus should bind less tight than multiplication but ...

- .. (slightly) stronger than addition and subtraction.

The latter condition ensures that "- x # ..." is resolved as "(- x) # ..." for any operator # with lower precedence than multiplication.

"-4 * 5" will be resolved to "-(4 * 5)" as Hugs correctly does already. I don't know how Helium resolves this, but it does (usually) not matter for multiplication if it were resolved to "(-4) * 5".

However, Hugs rejects "-4 # 5" for a non-left-associative operator # with precedence 6, because it compares associativities for operators with the same precedence. Therefore prefix minus should have a (slightly) higher precedence than infix operators with the precedence of infix minus.

infix # 6 (#) = (-) x7 = - 4 # 5

x7 are currently rejected by Hugs and ghc. Two solutions are possible. Since "-" should just bind weaker than multiplication the possibility "-(4 # 5)" cannot be justified, thus "(-4) # 5" should be the unique solution.

Surely, one can always disallow "confusing" resolutions, but if we reject "- 4 # 5", we can also reject "- 4 - 5" or "`- 4 ^ 5`".
Associativity seems wrong to consider for the unary minus function. (But it is an option to simple use the way of Hugs for ghc, too.)

As a further option it is possible to support multiple prefix minus application. ("4 * - - 5" can be resolved in the same way as "4 * - 5" is).

## References

For other examples see http://www.haskell.org/pipermail/haskell-prime/2010-July/003229.html

Mixfix analysis [Aasa95] usually only considers the top-level operators of argument terms. This backs my point 1. above: Do not look to the left of prefix minus, because there is no argument of prefix minus.

Also Isabelle (http://isabelle.in.tum.de/) allows prefix operators to have lower precedence than infix operators. Other specification languages (like HasCASL) do so, too. For instance the logical prefix negation binds stronger than logical connectives but weaker than infix equality or other comparisons. (This does not apply to the "not" in Haskell, because "not" is a plain function, no operator).

My point 3. is backed by the fact that "in case of doubt" prefix application should bind stronger than infix application.