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<title>src/full/Agda/Syntax/Parser/LookAhead.hs</title>
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<pre><a name="line-1"></a><span class='hs-comment'>{-# LANGUAGE Rank2Types #-}</span>
<a name="line-2"></a><span class='hs-comment'>{-| When lexing by hands (for instance string literals) we need to do some
<a name="line-3"></a> looking ahead. The 'LookAhead' monad keeps track of the position we are
<a name="line-4"></a> currently looking at, and provides facilities to synchronise the look-ahead
<a name="line-5"></a> position with the actual position of the 'Parser' monad (see 'sync' and
<a name="line-6"></a> 'rollback').
<a name="line-7"></a>-}</span>
<a name="line-8"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Agda</span><span class='hs-varop'>.</span><span class='hs-conid'>Syntax</span><span class='hs-varop'>.</span><span class='hs-conid'>Parser</span><span class='hs-varop'>.</span><span class='hs-conid'>LookAhead</span>
<a name="line-9"></a> <span class='hs-layout'>(</span> <span class='hs-comment'>-- * The LookAhead monad</span>
<a name="line-10"></a> <span class='hs-conid'>LookAhead</span>
<a name="line-11"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>runLookAhead</span>
<a name="line-12"></a> <span class='hs-comment'>-- * Operations</span>
<a name="line-13"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>getInput</span><span class='hs-layout'>,</span> <span class='hs-varid'>setInput</span><span class='hs-layout'>,</span> <span class='hs-varid'>liftP</span>
<a name="line-14"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>nextChar</span><span class='hs-layout'>,</span> <span class='hs-varid'>eatNextChar</span>
<a name="line-15"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>sync</span><span class='hs-layout'>,</span> <span class='hs-varid'>rollback</span>
<a name="line-16"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>match</span><span class='hs-layout'>,</span> <span class='hs-varid'>match'</span>
<a name="line-17"></a> <span class='hs-layout'>)</span>
<a name="line-18"></a> <span class='hs-keyword'>where</span>
<a name="line-19"></a>
<a name="line-20"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Reader</span>
<a name="line-21"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>State</span>
<a name="line-22"></a>
<a name="line-23"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Agda</span><span class='hs-varop'>.</span><span class='hs-conid'>Syntax</span><span class='hs-varop'>.</span><span class='hs-conid'>Parser</span><span class='hs-varop'>.</span><span class='hs-conid'>Alex</span>
<a name="line-24"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Agda</span><span class='hs-varop'>.</span><span class='hs-conid'>Syntax</span><span class='hs-varop'>.</span><span class='hs-conid'>Parser</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span>
<a name="line-25"></a>
<a name="line-26"></a><span class='hs-comment'>{--------------------------------------------------------------------------
<a name="line-27"></a> The look-ahead monad
<a name="line-28"></a> --------------------------------------------------------------------------}</span>
<a name="line-29"></a>
<a name="line-30"></a><a name="LookAhead"></a><span class='hs-comment'>{-| The LookAhead monad is basically a state monad keeping with an extra
<a name="line-31"></a> 'AlexInput', wrapped around the 'Parser' monad.
<a name="line-32"></a>-}</span>
<a name="line-33"></a><a name="LookAhead"></a><span class='hs-keyword'>newtype</span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span>
<a name="line-34"></a> <span class='hs-conid'>LookAhead</span> <span class='hs-layout'>{</span> <span class='hs-varid'>unLookAhead</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>ReaderT</span> <span class='hs-conid'>ErrorFunction</span>
<a name="line-35"></a> <span class='hs-layout'>(</span><span class='hs-conid'>StateT</span> <span class='hs-conid'>AlexInput</span> <span class='hs-conid'>Parser</span><span class='hs-layout'>)</span> <span class='hs-varid'>a</span>
<a name="line-36"></a> <span class='hs-layout'>}</span>
<a name="line-37"></a>
<a name="line-38"></a><a name="ErrorFunction"></a><span class='hs-keyword'>newtype</span> <span class='hs-conid'>ErrorFunction</span> <span class='hs-keyglyph'>=</span>
<a name="line-39"></a> <span class='hs-conid'>ErrorFun</span> <span class='hs-layout'>{</span> <span class='hs-varid'>throwError</span> <span class='hs-keyglyph'>::</span> <span class='hs-keyword'>forall</span> <span class='hs-varid'>a</span><span class='hs-varop'>.</span> <span class='hs-conid'>String</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>a</span> <span class='hs-layout'>}</span>
<a name="line-40"></a>
<a name="line-41"></a><span class='hs-comment'>{--------------------------------------------------------------------------
<a name="line-42"></a> Monad instances
<a name="line-43"></a> --------------------------------------------------------------------------}</span>
<a name="line-44"></a>
<a name="line-45"></a><a name="instance%20Monad%20LookAhead"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Monad</span> <span class='hs-conid'>LookAhead</span> <span class='hs-keyword'>where</span>
<a name="line-46"></a> <span class='hs-varid'>return</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>LookAhead</span> <span class='hs-varop'>.</span> <span class='hs-varid'>return</span>
<a name="line-47"></a> <span class='hs-varid'>m</span> <span class='hs-varop'>>>=</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>LookAhead</span> <span class='hs-varop'>$</span> <span class='hs-varid'>unLookAhead</span> <span class='hs-varid'>m</span> <span class='hs-varop'>>>=</span> <span class='hs-varid'>unLookAhead</span> <span class='hs-varop'>.</span> <span class='hs-varid'>k</span>
<a name="line-48"></a> <span class='hs-varid'>fail</span> <span class='hs-varid'>s</span> <span class='hs-keyglyph'>=</span>
<a name="line-49"></a> <span class='hs-keyword'>do</span> <span class='hs-varid'>err</span> <span class='hs-keyglyph'><-</span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>ask</span>
<a name="line-50"></a> <span class='hs-varid'>throwError</span> <span class='hs-varid'>err</span> <span class='hs-varid'>s</span>
<a name="line-51"></a>
<a name="line-52"></a><span class='hs-comment'>{--------------------------------------------------------------------------
<a name="line-53"></a> Operations
<a name="line-54"></a> --------------------------------------------------------------------------}</span>
<a name="line-55"></a>
<a name="line-56"></a><a name="getInput"></a><span class='hs-comment'>-- | Get the current look-ahead position.</span>
<a name="line-57"></a><span class='hs-definition'>getInput</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>LookAhead</span> <span class='hs-conid'>AlexInput</span>
<a name="line-58"></a><span class='hs-definition'>getInput</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>get</span>
<a name="line-59"></a>
<a name="line-60"></a>
<a name="line-61"></a><a name="setInput"></a><span class='hs-comment'>-- | Set the look-ahead position.</span>
<a name="line-62"></a><span class='hs-definition'>setInput</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>AlexInput</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>LookAhead</span> <span class='hs-conid'>()</span>
<a name="line-63"></a><span class='hs-definition'>setInput</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>LookAhead</span> <span class='hs-varop'>.</span> <span class='hs-varid'>put</span>
<a name="line-64"></a>
<a name="line-65"></a>
<a name="line-66"></a><a name="liftP"></a><span class='hs-comment'>-- | Lift a computation in the 'Parser' monad to the 'LookAhead' monad.</span>
<a name="line-67"></a><span class='hs-definition'>liftP</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Parser</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>a</span>
<a name="line-68"></a><span class='hs-definition'>liftP</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>LookAhead</span> <span class='hs-varop'>.</span> <span class='hs-varid'>lift</span> <span class='hs-varop'>.</span> <span class='hs-varid'>lift</span>
<a name="line-69"></a>
<a name="line-70"></a>
<a name="line-71"></a><a name="nextChar"></a><span class='hs-comment'>-- | Look at the next character. Fails if there are no more characters.</span>
<a name="line-72"></a><span class='hs-definition'>nextChar</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>LookAhead</span> <span class='hs-conid'>Char</span>
<a name="line-73"></a><span class='hs-definition'>nextChar</span> <span class='hs-keyglyph'>=</span>
<a name="line-74"></a> <span class='hs-keyword'>do</span> <span class='hs-varid'>inp</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>getInput</span>
<a name="line-75"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>alexGetChar</span> <span class='hs-varid'>inp</span> <span class='hs-keyword'>of</span>
<a name="line-76"></a> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>fail</span> <span class='hs-str'>"unexpected end of file"</span>
<a name="line-77"></a> <span class='hs-conid'>Just</span> <span class='hs-layout'>(</span><span class='hs-varid'>c</span><span class='hs-layout'>,</span><span class='hs-varid'>inp'</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-78"></a> <span class='hs-keyword'>do</span> <span class='hs-varid'>setInput</span> <span class='hs-varid'>inp'</span>
<a name="line-79"></a> <span class='hs-varid'>return</span> <span class='hs-varid'>c</span>
<a name="line-80"></a>
<a name="line-81"></a>
<a name="line-82"></a><a name="sync"></a><span class='hs-comment'>-- | Consume all the characters up to the current look-ahead position.</span>
<a name="line-83"></a><span class='hs-definition'>sync</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>LookAhead</span> <span class='hs-conid'>()</span>
<a name="line-84"></a><span class='hs-definition'>sync</span> <span class='hs-keyglyph'>=</span>
<a name="line-85"></a> <span class='hs-keyword'>do</span> <span class='hs-varid'>inp</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>getInput</span>
<a name="line-86"></a> <span class='hs-varid'>liftP</span> <span class='hs-varop'>$</span> <span class='hs-varid'>setLexInput</span> <span class='hs-varid'>inp</span>
<a name="line-87"></a>
<a name="line-88"></a>
<a name="line-89"></a><a name="rollback"></a><span class='hs-comment'>-- | Undo look-ahead. Restores the input from the 'ParseState'.</span>
<a name="line-90"></a><span class='hs-definition'>rollback</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>LookAhead</span> <span class='hs-conid'>()</span>
<a name="line-91"></a><span class='hs-definition'>rollback</span> <span class='hs-keyglyph'>=</span>
<a name="line-92"></a> <span class='hs-keyword'>do</span> <span class='hs-varid'>inp</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>liftP</span> <span class='hs-varid'>getLexInput</span>
<a name="line-93"></a> <span class='hs-varid'>setInput</span> <span class='hs-varid'>inp</span>
<a name="line-94"></a>
<a name="line-95"></a>
<a name="line-96"></a><a name="eatNextChar"></a><span class='hs-comment'>-- | Consume the next character. Does 'nextChar' followed by 'sync'.</span>
<a name="line-97"></a><span class='hs-definition'>eatNextChar</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>LookAhead</span> <span class='hs-conid'>Char</span>
<a name="line-98"></a><span class='hs-definition'>eatNextChar</span> <span class='hs-keyglyph'>=</span>
<a name="line-99"></a> <span class='hs-keyword'>do</span> <span class='hs-varid'>c</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>nextChar</span>
<a name="line-100"></a> <span class='hs-varid'>sync</span>
<a name="line-101"></a> <span class='hs-varid'>return</span> <span class='hs-varid'>c</span>
<a name="line-102"></a>
<a name="line-103"></a>
<a name="line-104"></a><a name="match"></a><span class='hs-comment'>{-| Do a case on the current input string. If any of the given strings match we
<a name="line-105"></a> move past it and execute the corresponding action. If no string matches, we
<a name="line-106"></a> execute a default action, advancing the input one character. This function
<a name="line-107"></a> only affects the look-ahead position.
<a name="line-108"></a>-}</span>
<a name="line-109"></a><span class='hs-definition'>match</span> <span class='hs-keyglyph'>::</span> <span class='hs-keyglyph'>[</span><span class='hs-layout'>(</span><span class='hs-conid'>String</span><span class='hs-layout'>,</span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>a</span>
<a name="line-110"></a><span class='hs-definition'>match</span> <span class='hs-varid'>xs</span> <span class='hs-varid'>def</span> <span class='hs-keyglyph'>=</span>
<a name="line-111"></a> <span class='hs-keyword'>do</span> <span class='hs-varid'>c</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>nextChar</span>
<a name="line-112"></a> <span class='hs-varid'>match'</span> <span class='hs-varid'>c</span> <span class='hs-varid'>xs</span> <span class='hs-varid'>def</span>
<a name="line-113"></a>
<a name="line-114"></a><a name="match'"></a><span class='hs-comment'>{-| Same as 'match' but takes the initial character from the first argument
<a name="line-115"></a> instead of reading it from the input. Consequently, in the default case
<a name="line-116"></a> the input is not advanced.
<a name="line-117"></a>-}</span>
<a name="line-118"></a><span class='hs-definition'>match'</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Char</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-layout'>(</span><span class='hs-conid'>String</span><span class='hs-layout'>,</span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>a</span>
<a name="line-119"></a><span class='hs-definition'>match'</span> <span class='hs-varid'>c</span> <span class='hs-varid'>xs</span> <span class='hs-varid'>def</span> <span class='hs-keyglyph'>=</span>
<a name="line-120"></a> <span class='hs-keyword'>do</span> <span class='hs-varid'>inp</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>getInput</span>
<a name="line-121"></a> <span class='hs-varid'>match''</span> <span class='hs-varid'>inp</span> <span class='hs-varid'>xs</span> <span class='hs-varid'>c</span>
<a name="line-122"></a> <span class='hs-keyword'>where</span>
<a name="line-123"></a> <span class='hs-varid'>match''</span> <span class='hs-varid'>inp</span> <span class='hs-varid'>bs</span> <span class='hs-varid'>c</span> <span class='hs-keyglyph'>=</span>
<a name="line-124"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>bs'</span> <span class='hs-keyword'>of</span>
<a name="line-125"></a> <span class='hs-conid'>[]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>setInput</span> <span class='hs-varid'>inp</span> <span class='hs-varop'>>></span> <span class='hs-varid'>def</span>
<a name="line-126"></a> <span class='hs-keyglyph'>[</span><span class='hs-layout'>(</span><span class='hs-str'>""</span><span class='hs-layout'>,</span><span class='hs-varid'>p</span><span class='hs-layout'>)</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>p</span>
<a name="line-127"></a> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>match''</span> <span class='hs-varid'>inp</span> <span class='hs-varid'>bs'</span> <span class='hs-varop'>=<<</span> <span class='hs-varid'>nextChar</span>
<a name="line-128"></a> <span class='hs-keyword'>where</span>
<a name="line-129"></a> <span class='hs-varid'>bs'</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyglyph'>[</span> <span class='hs-layout'>(</span><span class='hs-varid'>s</span><span class='hs-layout'>,</span> <span class='hs-varid'>p</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>|</span> <span class='hs-layout'>(</span><span class='hs-varid'>c'</span><span class='hs-conop'>:</span><span class='hs-varid'>s</span><span class='hs-layout'>,</span> <span class='hs-varid'>p</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>bs</span><span class='hs-layout'>,</span> <span class='hs-varid'>c</span> <span class='hs-varop'>==</span> <span class='hs-varid'>c'</span> <span class='hs-keyglyph'>]</span>
<a name="line-130"></a>
<a name="line-131"></a><a name="runLookAhead"></a><span class='hs-comment'>-- | Run a 'LookAhead' computation. The first argument is the error function.</span>
<a name="line-132"></a><span class='hs-definition'>runLookAhead</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-keyword'>forall</span> <span class='hs-varid'>b</span><span class='hs-varop'>.</span> <span class='hs-conid'>String</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>LookAhead</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Parser</span> <span class='hs-varid'>a</span>
<a name="line-133"></a><span class='hs-definition'>runLookAhead</span> <span class='hs-varid'>err</span> <span class='hs-layout'>(</span><span class='hs-conid'>LookAhead</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span>
<a name="line-134"></a> <span class='hs-keyword'>do</span> <span class='hs-varid'>inp</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>getLexInput</span>
<a name="line-135"></a> <span class='hs-varid'>evalStateT</span> <span class='hs-layout'>(</span><span class='hs-varid'>runReaderT</span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>ErrorFun</span> <span class='hs-varid'>err</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-varid'>inp</span>
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