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<p>
Up until now each function has accepted precisely one type for each of its
arguments.
</p>
<p>
The <code>twice</code> function for example only worked with functions that
would take and return ints. This is overly restrictive, it should be possible
to use this function with any type, so long as the function and the initial
value are compatible.
</p>
<p>
To enable this Gleam support <em>generics</em>, also known as
<em>parametric polymorphism</em>.
</p>
<p>
This works by instead of specifying a concrete type, a type variable is used
which stands in for whatever specific type is being used when the function is
called. These type variables are written with a lowercase name.
</p>
<p>
Type variables are not like an <code>any</code> type, they get replaced with a
specific type each time the function is called. Try uncommenting
<code>twice(10, exclaim)</code> to see the compiler error from trying to use a
type variable as an int and a string at the same time.
</p>
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