What does the <code>#</code> token mean and how is it used? <pre class="prettyprint"><code>newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #)) </code></pre> I came across this here and wonder where this type is actually defined or weather this is some syntax of a internal type that is ultimately defined in C?

The <code>#</code> marks the type as an unboxed type. The <code>#</code> is conventional, but requires an extension (<code>MagicHash</code>) to make it valid syntax. Likewise, <code>(# ... #)</code> is the syntax for an unboxed tuple.

Haskell token "#"

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haskell

What does the # token mean and how is it used?

newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))

I came across this here and wonder where this type is actually defined or weather this is some syntax of a internal type that is ultimately defined in C?

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asked May 04 '19 11:05

Konrad Eisele

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The # marks the type as an unboxed type. The # is conventional, but requires an extension (MagicHash) to make it valid syntax. Likewise, (# ... #) is the syntax for an unboxed tuple.