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<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">You could use DataKinds with GADTs like so</p>
<p style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; "> </p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';">{-# LANGUAGE DataKinds #-}</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';">{-# LANGUAGE KindSignatures #-}</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';">{-# LANGUAGE GADTs #-}</span></p>
<p style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; "> </p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';">data UserType = RealUser | VirtualUser</span></p>
<p style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; "> </p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';">data User (userType :: UserType) where</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';"> MkRealUser :: Address -> User RealUser</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';"> MkVirtualUser :: Email -> User VirtualUser</span></p>
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<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';">deliverPackage :: User RealUser -> Package -> IO Bool</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';">deliverPackage user = case user of</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';"> -- This is the only valid pattern, since MkVritualUser would imply</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';"> -- the type User VirtualUser. </span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';"> MkRealUser address -> ...</span></p>
<p style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; "> </p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';">sendEmail :: User VirtualUser -> EmailMsg -> IO ()</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';">sendEmail user = case user of</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace';"> MkVritualUser email -> ...</span></p>
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<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">Alex</p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br />On June 11, 2015 09:54:54 AM Román González wrote:<br /></p>
<p style=" margin-top:12px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">Hello there,</p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br /></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">I've been thinking on different approaches to constraint particular functions to a particular constructor of an ADT in order to reduce representation of invalid states. Say for example I've this types:</p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br /></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace, monospace';">data Address = Address { ... }</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace, monospace';">newtype Email = Email String</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace, monospace';">data Package = Package { ... }</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace, monospace';">data EmailMsg = EmailMsg { ... }</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br /></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace, monospace';">data User </span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace, monospace';"> = RealUser Address</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace, monospace';"> | VirtualUser Email</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br /></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">And I would like to implement two functions:</p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br /></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace, monospace';">deliverPackage :: User -> Package -> IO Bool</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><span style=" font-family:'monospace, monospace';">sendEmail :: User -> EmailMsg -> IO ()</span></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br /></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">I would like to constraint both <span style=" font-family:'monospace, monospace';">deliverPackage</span> and <span style=" font-family:'monospace, monospace';">sendEmail</span> to receive only the semantically correct constructor of <span style=" font-family:'monospace, monospace';">User</span>. </p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br /></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">I know of an approach I could use, that is wrapping each constructor in it's own<span style=" font-family:'monospace, monospace';"> newtype</span>, and create some smart constructor that way, that approach works, but I find it rather verbose. </p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br /></p>
<p style=" margin-top:0px; margin-bottom:0px; margin-left:40px; margin-right:40px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">Is there any other well known approach to deal with this scenarios?</p>
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