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Implement the proposal of underscores in numeric literals. Underscores in numeric literals are simply ignored. The specification of the feature is available here: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/000 9-numeric-underscores.rst For a discussion of the various choices: https://github.com/ghc-proposals/ghc-proposals/pull/76 Implementation detail: * Added dynamic flag * `NumericUnderscores` extension flag is added for this feature. * Alex "Regular expression macros" in Lexer.x * Add `@numspc` (numeric spacer) macro to represent multiple underscores. * Modify `@decimal`, `@decimal`, `@binary`, `@octal`, `@hexadecimal`, `@exponent`, and `@bin_exponent` macros to include `@numspc`. * Alex "Rules" in Lexer.x * To be simpler, we have only the definitions with underscores. And then we have a separate function (`tok_integral` and `tok_frac`) that validates the literals. * Validation functions in Lexer.x * `tok_integral` and `tok_frac` functions validate whether contain underscores or not. If `NumericUnderscores` extensions are not enabled, check that there are no underscores. * `tok_frac` function is created by merging `strtoken` and `init_strtoken`. * `init_strtoken` is deleted. Because it is no longer used. * Remove underscores from target literal string * `parseUnsignedInteger`, `readRational__`, and `readHexRational} use the customized `span'` function to remove underscores. * Added Testcase * testcase for NumericUnderscores enabled. NumericUnderscores0.hs and NumericUnderscores1.hs * testcase for NumericUnderscores disabled. NoNumericUnderscores0.hs and NoNumericUnderscores1.hs * testcase to invalid pattern for NumericUnderscores enabled. NumericUnderscoresFail0.hs and NumericUnderscoresFail1.hs Test Plan: `validate` including the above testcase Reviewers: goldfire, bgamari Reviewed By: bgamari Subscribers: carter, rwbarton, thomie GHC Trac Issues: #14473 Differential Revision: https://phabricator.haskell.org/D4235
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