diff --git a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/doubles/doubles.kt b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/doubles/doubles.kt
index 4479511..9114ad7 100644
--- a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/doubles/doubles.kt
+++ b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/doubles/doubles.kt
@@ -5,7 +5,6 @@ import arrow.core.right
 import com.sksamuel.tribune.core.Parser
 import com.sksamuel.tribune.core.filter
 import com.sksamuel.tribune.core.flatMap
-import com.sksamuel.tribune.core.map
 
 /**
  * Extends a [Parser] of output type string to parse that string into a double.
@@ -44,10 +43,3 @@ fun <I, E> Parser<I, Double, E>.inrange(
    flatMap {
       if (it in range) it.right() else ifError(it).leftNel()
    }
-
-fun <I, E> Parser<I, Double, E>.nullIf(fn: (Double) -> Boolean): Parser<I, Double?, E> =
-   this.map { if (fn(it)) null else it }
-
-@JvmName("nullIfNullable")
-fun <I, E> Parser<I, Double?, E>.nullIf(fn: (Double) -> Boolean): Parser<I, Double?, E> =
-   this.map { if (it == null || fn(it)) null else it }
diff --git a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/filter.kt b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/filter.kt
index bd8500a..f6fa80f 100644
--- a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/filter.kt
+++ b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/filter.kt
@@ -17,6 +17,36 @@ import arrow.core.right
  *
  * @return a parser which rejects input based on the result of predicate [p]
  */
-fun <I, A, E> Parser<I, A, E>.filter(p: (A) -> Boolean, ifFalse: (A) -> E): Parser<I, A, E> {
+fun <I, O, E> Parser<I, O, E>.filter(p: (O) -> Boolean, ifFalse: (O) -> E): Parser<I, O, E> {
    return flatMap { if (p(it)) it.right() else ifFalse(it).leftNel() }
 }
+
+/**
+ * Returns a [Parser] that produces a null if the input value fails to pass the predicate [p].
+ *
+ * In other words, if the underlying parser returns a valid output, that output is then
+ * passed to the given predicate, and if that predicate returns false, a null is produced.
+ *
+ * @param p the predicate to test input
+ *
+ * @return a parser which rejects input based on the result of predicate [p]
+ */
+fun <I, O : Any, E> Parser<I, O, E>.nullIf(fn: (O) -> Boolean): Parser<I, O?, E> =
+   this.map { if (fn(it)) null else it }
+
+/**
+ * Returns a [Parser] that produces a null if the input value fails to pass the predicate [p].
+ *
+ * In other words, if the underlying parser returns a valid output, that output is then
+ * passed to the given predicate, and if that predicate returns false, a null is produced.
+ *
+ * This variant of [nullIf] allows the output of the preceeding parser to already be null.
+ *
+ * @param p the predicate to test input
+ *
+ * @return a parser which rejects input based on the result of predicate [p]
+ */
+@JvmName("nullIfNullable")
+fun <I, O, E> Parser<I, O?, E>.nullIf(fn: (O) -> Boolean): Parser<I, O?, E> =
+   this.map { if (it == null || fn(it)) null else it }
+
diff --git a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/floats/floats.kt b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/floats/floats.kt
index b4dd833..f1637d3 100644
--- a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/floats/floats.kt
+++ b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/floats/floats.kt
@@ -4,7 +4,6 @@ import arrow.core.leftNel
 import arrow.core.right
 import com.sksamuel.tribune.core.Parser
 import com.sksamuel.tribune.core.flatMap
-import com.sksamuel.tribune.core.map
 
 /**
  * Extends a [Parser] of output type string to parse that string into a double.
@@ -19,10 +18,3 @@ fun <I, E> Parser<I, String, E>.float(ifError: (String) -> E): Parser<I, Float,
       val f = it.toFloatOrNull()
       f?.right() ?: ifError(it).leftNel()
    }
-
-fun <I, E> Parser<I, Float, E>.nullIf(fn: (Float) -> Boolean): Parser<I, Float?, E> =
-   this.map { if (fn(it)) null else it }
-
-@JvmName("nullIfNullable")
-fun <I, E> Parser<I, Float?, E>.nullIf(fn: (Float) -> Boolean): Parser<I, Float?, E> =
-   this.map { if (it == null || fn(it)) null else it }
diff --git a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/ints/ints.kt b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/ints/ints.kt
index c6a9089..911633e 100644
--- a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/ints/ints.kt
+++ b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/ints/ints.kt
@@ -6,7 +6,6 @@ import arrow.core.right
 import com.sksamuel.tribune.core.Parser
 import com.sksamuel.tribune.core.filter
 import com.sksamuel.tribune.core.flatMap
-import com.sksamuel.tribune.core.map
 
 /**
  * Chains a [Parser] to convert String -> Int.
@@ -61,10 +60,3 @@ fun <I, E> Parser<I, Int?, E>.max(min: Int, ifError: (Int) -> E): Parser<I, Int?
    flatMap {
       if (it == null) Either.Right(null) else if (it >= min) it.right() else ifError(it).leftNel()
    }
-
-fun <I, E> Parser<I, Int, E>.nullIf(fn: (Int) -> Boolean): Parser<I, Int?, E> =
-   this.map { if (fn(it)) null else it }
-
-@JvmName("nullIfNullable")
-fun <I, E> Parser<I, Int?, E>.nullIf(fn: (Int) -> Boolean): Parser<I, Int?, E> =
-   this.map { if (it == null || fn(it)) null else it }
diff --git a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/longs/longs.kt b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/longs/longs.kt
index 321f896..dfe651c 100644
--- a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/longs/longs.kt
+++ b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/longs/longs.kt
@@ -47,10 +47,3 @@ fun <I, E> Parser<I, Long?, E>.max(min: Long, ifError: (Long) -> E): Parser<I, L
    flatMap {
       if (it == null) Either.Right(null) else if (it >= min) it.right() else ifError(it).leftNel()
    }
-
-fun <I, E> Parser<I, Long, E>.nullIf(fn: (Long) -> Boolean): Parser<I, Long?, E> =
-   this.map { if (fn(it)) null else it }
-
-@JvmName("nullIfNullable")
-fun <I, E> Parser<I, Long?, E>.nullIf(fn: (Long) -> Boolean): Parser<I, Long?, E> =
-   this.map { if (it == null || fn(it)) null else it }
diff --git a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/strings/strings.kt b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/strings/strings.kt
index 71227a2..ee2f7cf 100644
--- a/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/strings/strings.kt
+++ b/tribune-core/src/main/kotlin/com/sksamuel/tribune/core/strings/strings.kt
@@ -77,10 +77,3 @@ fun <I, E> Parser<I, String?, E>.nullOrNotBlank(ifBlank: () -> E): Parser<I, Str
       if (it == null) null.right() else if (it.isBlank()) ifBlank().leftNel() else it.right()
    }
 }
-
-fun <I, E> Parser<I, String, E>.nullIf(fn: (String) -> Boolean): Parser<I, String?, E> =
-   this.map { if (fn(it)) null else it }
-
-@JvmName("nullIfNullable")
-fun <I, E> Parser<I, String?, E>.nullIf(fn: (String) -> Boolean): Parser<I, String?, E> =
-   this.map { if (it == null || fn(it)) null else it }
diff --git a/tribune-core/src/test/kotlin/com/sksamuel/tribune/core/ComposeTest.kt b/tribune-core/src/test/kotlin/com/sksamuel/tribune/core/ComposeTest.kt
index 921d562..0ea310a 100644
--- a/tribune-core/src/test/kotlin/com/sksamuel/tribune/core/ComposeTest.kt
+++ b/tribune-core/src/test/kotlin/com/sksamuel/tribune/core/ComposeTest.kt
@@ -1,6 +1,5 @@
 package com.sksamuel.tribune.core
 
-import com.sksamuel.tribune.core.doubles.nullIf
 import com.sksamuel.tribune.core.ints.min
 import io.kotest.core.spec.style.FunSpec
 import io.kotest.matchers.shouldBe
@@ -39,6 +38,8 @@ class ComposeTest : FunSpec() {
    }
 }
 
+val q = Parsers.nullableString.nullIf { it.length > 3 }
+
 val weightParser = Parsers.nullableDouble
    .nullIf { it <= 0.0 }
    .mapIfNotNull { ParsedWeight(it) }
diff --git a/tribune-core/src/test/kotlin/com/sksamuel/tribune/core/NullTest.kt b/tribune-core/src/test/kotlin/com/sksamuel/tribune/core/NullTest.kt
index 11ed6d8..08600b7 100644
--- a/tribune-core/src/test/kotlin/com/sksamuel/tribune/core/NullTest.kt
+++ b/tribune-core/src/test/kotlin/com/sksamuel/tribune/core/NullTest.kt
@@ -3,7 +3,6 @@ package com.sksamuel.tribune.core
 import arrow.core.Either
 import arrow.core.leftNel
 import arrow.core.right
-import com.sksamuel.tribune.core.strings.nullIf
 import io.kotest.core.spec.style.FunSpec
 import io.kotest.matchers.shouldBe
 
@@ -15,6 +14,19 @@ class NullTest : FunSpec() {
          p.parse("abc") shouldBe "wibble".right()
       }
 
+      test("nullIf on O -> A?") {
+         val p = Parser<String>().nullIf { "foo" == it }
+         p.parse("foo") shouldBe null.right()
+         p.parse("bar") shouldBe "bar".right()
+      }
+
+      test("nullIf on O? -> A?") {
+         val p = Parser<String?>().nullIf { "foo" == it }
+         p.parse(null) shouldBe null.right()
+         p.parse("foo") shouldBe null.right()
+         p.parse("bar") shouldBe "bar".right()
+      }
+
       test("withDefault on I? -> A?") {
          val p = Parser<String?>().withDefault { "wibble" }
          p.parse("abc") shouldBe "abc".right()