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Haskell backend: happy does not accept empty token list, so bnfc should check for it #213

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andreasabel opened this issue Nov 7, 2017 · 3 comments
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Haskell backend: happy does not accept empty token list, so bnfc should check for it #213

andreasabel opened this issue Nov 7, 2017 · 3 comments
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enhancement Haskell LBNF Concerning the LBNF syntax and its checking parser Issues concerning parser generating
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2.8.4

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@andreasabel
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This (evil) grammar fails with a parse error in the Par.y file raised by happy:

A. S := T;
B. T := S;

happy does not like empty %token list.
@bafain
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bafain commented Nov 14, 2018

Although the user guide says that the %token directive is mandatory, happy 1.19.9 does not complain if we omit it.

@andreasabel andreasabel self-assigned this Feb 7, 2019
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andreasabel commented Feb 7, 2019 •
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In case of no tokens, happy now does not output a %token declaration: f42310d.
Works for Haskell and Java backends now.

C/CPP: bison complains:

bison -t -pIssue213 Issue213.y -o Parser.c
Issue213.y: warning: 3 nonterminals useless in grammar [-Wother]
Issue213.y: warning: 3 rules useless in grammar [-Wother]
Issue213.y:68.8: fatal error: start symbol S does not derive any sentence
 %start S
        ^

I think this issue is not worth pursuing further.

@andreasabel andreasabel added enhancement LBNF Concerning the LBNF syntax and its checking parser Issues concerning parser generating and removed abandoned labels Oct 8, 2020
@andreasabel andreasabel added this to the 2.8.4 milestone Oct 8, 2020
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It is easy to insert a check whether any terminals appear in the grammar.

@andreasabel andreasabel reopened this Oct 8, 2020
andreasabel added a commit that referenced this issue Oct 8, 2020
@andreasabel
[ #213 ] correct the check for empty language
608745d 
The test in 84e19a0 was not correct
since it ignored user-defined token categories in the grammar.
If one of these appears on a rhs, the language may not be empty.
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enhancement Haskell LBNF Concerning the LBNF syntax and its checking parser Issues concerning parser generating
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