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# Conditionals and `do`-notation
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## Brief Explanation
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<table><tr><th> Ticket </th>
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<th>
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<table><tr><th>[\#23](https://gitlab.haskell.org//haskell/prime/issues/23)</th>
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<td>DoAndIfThenElse</td></tr></table>
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Dependencies Related [NondecreasingIndentation](nondecreasing-indentation)Compiler support GHC full (no flag)
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nhc98 full (no flag)
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Hugs full (no flag)
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UHC full (no flag)
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JHC full (no flag)
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LHC full (no flag)
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Brief Explanation
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I have lost count of how many times people (including very experienced programmers) have approached me wondering why on earth code like the
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following fails to parse:
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```wiki
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foo :: Monad m => Bool -> m Int
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foo :: Monad m =\> Bool -\> m Int
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foo x = do
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if x then
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return 1
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else
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return 2
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```
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When I explain that they have to indent the `else` at least one more step, they invariably stare at me in disbelief and vow to henceforth
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stay far away from languages where layout has semantic significance. It can actually be somewhat embarassing, and it certainly can
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put beginners off.
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Strictly speaking, the issue is not tied to the `do`-notation, but that is where it (almost?) always shows up.
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## Proposal
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Proposal
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Change the syntax for conditionals to
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>
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> exp → `if` exp<sub>1</sub> \[`;`\] `then` exp<sub>2</sub> \[`;`\] `else` exp<sub>3</sub>
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exp → `if` exp<sub>1</sub> \[`;`\] `then` exp<sub>2</sub> \[`;`\] `else` exp<sub>3</sub>
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i.e., add optional semicolons before `then` and `else`, making the above example legal.
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This has been recently added to jhc, GHC and Hugs, and so far it has not caused any problems.
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The second optional semicolon is the important one.
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The first one just adds a little more convenience.
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**Possible addition** (this addition is **not** part of the proposal as accepted for Haskell 2010): Jon Fairbairn suggests that if an optional semicolon is added before `then`, it would be consistent to also add one before `of` in `case` expressions, allowing:
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```wiki
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do case whatever
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of
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blah -> bluh
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blah' -> bluh'
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```
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## References
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- [ Conditionals](http://www.haskell.org/onlinereport/exps.html#conditionals) in the Haskell 98 Report
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- Somewhat related to [NondecreasingIndentation](nondecreasing-indentation)
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## Tickets
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<table><tr><th>[\#23](https://gitlab.haskell.org//haskell/prime/issues/23)</th>
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<td>DoAndIfThenElse</td></tr></table>
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## Pros
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- Would address a layout issue that manifestly trips up a lot of people, and many experienced Haskell programmers find very annoying.
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- Trivial to implement
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- Independent of the layout rule.
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## Cons
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- It has been pointed out that improved error reporting could alleviate this issue to some extent, without
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any language change. An alternative might thus be to add a section to the report with recommendations
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concerning "best practices" for reporting errors related to this issue and other that are known to
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cause confusion.
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- People might actually write these optional semicolons literally
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in their code, thus relying on this hack, making it harder to fix the problem in a better way later. |
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blah -\> bluh
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blah' -\> bluh'
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References[ Conditionals](http://www.haskell.org/onlinereport/exps.html#conditionals) in the Haskell 98 Report
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Somewhat related to [NondecreasingIndentation](nondecreasing-indentation)ProsWould address a layout issue that manifestly trips up a lot of people, and many experienced Haskell programmers find very annoying.
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Trivial to implement
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Independent of the layout rule.
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ConsIt has been pointed out that improved error reporting could alleviate this issue to some extent, without
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any language change. An alternative might thus be to add a section to the report with recommendations
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concerning "best practices" for reporting errors related to this issue and other that are known to
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cause confusion.
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People might actually write these optional semicolons literally
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in their code, thus relying on this hack, making it harder to fix the problem in a better way later.
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Download in other formats:[Plain Text](/wiki/DoAndIfThenElse?version=15&format=txt)[](http://trac.edgewall.org/)Powered by [Trac 1.2.2](/about)
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By [Edgewall Software](http://www.edgewall.org/).Visit the Trac open source project at
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[http://trac.edgewall.com/](http://trac.edgewall.com/)
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</th></tr></table> |
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\ No newline at end of file |
